<PAGE>   1
                                                                   EXHIBIT 10.99

                          NAFI OWNER TRUSTEE AND NFAFT
                                INDENTURE TRUSTEE
                         IRREVOCABLE INSTRUCTION LETTER

                                                    As of March 31, 1999

Wilmington Trust Company, as Owner Trustee
1100 North Market Street
Wilmington, DE 19890
Attn: Corporate Trust Administration

Harris Trust and Savings Bank, as Trustee,  Chase Manhattan Bank Delaware,
Collateral Agent and Back-up Servicer       as Trustee
311 West Monroe Street, 12th Floor          1201 Market Street 9th Floor
Chicago, IL 60606                           Wilmington, DE 19801
Attn: Indenture Trust Division              Attn: Corporate Trust Administration

     Re:  (1)  First Amended and Restated Trust Agreement, dated as of December
               8, 1994, together with the Amendment thereto, dated as of
               November 21, 1995, and as amended October 1, 1997 and March 31,
               1997 among National Auto Finance Company L.P. ("NAFCO"), Chase
               Manhattan Bank Delaware (as successor to Chase Manhattan Bank
               USA, N.A.) ("Chase"), as Trustee (not in its individual capacity
               but solely as trustee, the "NFAFT Owner Trustee") ("NFAFT Trust
               Agreement").

          (2)  The Pooling and Servicing Agreement, dated as of October 1, 1995
               (the "95-1 Pooling and Servicing Agreement"), among National
               Financial Auto Funding Trust ("NFAFT"), as Transferor, NAFCO, as
               Master Servicer, and Harris Trust and Savings Bank, as Trustee
               and Collateral Agent ("Harris").

          (3)  The Pooling and Servicing Agreement, dated as of October 21, 1996
               (the "96-1 Pooling and Servicing Agreement"), among NFAFT, NAFCO,
               as Servicer, and Harris, as Trustee and Collateral Agent.

          (4)  Sale and Servicing Agreement, dated as of June 29, 1997 (the
               "97-1 Sale and Servicing Agreement"), among National Auto Finance
               1997-1 Trust, as Issuer (the "1997-1 Owner Trust"), NFAFT, as
               Seller, National Auto Finance Company, Inc., ("NAFI") as
               Servicer, and Harris, as Trust Collateral Agent and Back-up
               Servicer.

          (5)  Indenture, dated as of June 29, 1997, between National Auto
               Finance 1997-1 Trust and Harris, as Indenture Trustee ("97-1
               Indenture").


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          (6)  Trust Agreement, dated as of July 21, 1997, between NFAFT and
               Wilmington Trust Company ("Wilmington"), as Owner Trustee (not
               in its individual capacity but solely as trustee, the "1997-1
               Owner Trustee") ("1997-1 Trust Agreement").

          (7)  Sale and Servicing Agreement, dated as of December 15, 1997 (the
               "98-1 Sale and Servicing Agreement"), among National Auto Finance
               1998-1 Trust, as Issuer (the "1998-1 Owner Trust"), NFAFT, as
               Seller, NAFI, as Servicer, and Harris, as Trust Collateral Agent
               and Back-up Servicer.

          (8)  Indenture, dated as of December 15, 1997, between National Auto
               Finance 1998-1 Trust, as Issuer, and Harris, as Indenture Trustee
               ("98-1 Indenture").

          (9)  Trust Agreement, dated as of December 15, 1997, between NFAFT and
               Wilmington, as Owner Trustee (not in its individual capacity but
               solely as trustee, the "1998-1 Owner Trustee") ("1998-1 Trust
               Agreement").

          (10) Custodian Agreement, dated as of October 15, 1998, together with
               the Amendment to the Custodian Agreement dated as of the date
               hereof, among Harris, as Custodian, NAFI, and Harris, as Trustee
               pursuant to the 95-1 Pooling and Servicing Agreement and the 96-1
               Pooling and Servicing Agreement, Trust Collateral Agent pursuant
               to the 97-1 Sale and Servicing Agreement, the 98-1 Sale and
               Servicing Agreement, the 97-1 Indenture and the 98-1 Indenture.

          (11) First Amended and Restated Master Spread Account Agreement, dated
               as of March 31, 1998 among Harris, NFAFT and FSA, and the
               supplements thereto (the "Master Spread Account Agreement").


Ladies and Gentlemen:

     Reference is made to: (a) the NFAFT Trust Agreement, (b) the Subordination
and Intercreditor Agreement, by and among National Auto Finance 1995-1 Trust,
National Auto Finance 1996-1 Trust, National Auto Finance 1997-1 Trust and
National Auto Finance 1998-1 Trust (collectively, the "NAFI Trusts"), NFAFT,
NAFI, First Union National Bank ("FUNB"), Financial Security Assurance Inc.
("FSA") and Harris (the "Intercreditor Agreement") and (c) the other foregoing
agreements listed above in items 1 through 9 together with all amendments,
modifications and supplements thereto (the "Pooling Agreements," and, together
with the other agreements, indentures, documents and instruments related
thereto, the "Underlying Agreements"), pursuant to which the NFAFT Owner Trustee
holds the trust estate, as described in the NFAFT Trust Agreement, for the use
and benefit of the beneficiary(ies) of NFAFT subject to the provisions of the
liens created by the Underlying Agreements; and, pursuant to such Underlying
Agreements, Harris holds the trust estate comprised of the NAFI Trusts for the
benefit of certain securityholders, FSA and NFAFT. The signatories hereto may
execute in counterparts, in which case, each executed copy of this Direction
Letter shall constitute an original, and all of which when taken together shall
constitute one original Direction Letter.


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     A.   NOTICE AND INSTRUCTIONS FROM NAFI AND BORROWER

          1.   Notice of Pledge. Each of you is hereby notified that pursuant to
the Revolving Credit, Term Loan and Security Agreement (the "Credit Agreement")
to be entered into by and among the FUNB, NAFI and NFAFT, that (i) NFAFT and
NAFI will pledge all of its right, title and interest in and to payments and
distributions on each Distribution Date (i) to which NFAFT and NAFI would
otherwise be entitled under and pursuant to the following agreements with
respect to the following transactions: (a) in the case of the Trust Property of
the 1995-1 transaction, subsection (xi) of Section 4.01 of the 1995-1 Pooling
and Servicing Agreement; (b) in the case of the Trust Property of the 1996-1
transaction, subsection (xi) of Section 4.01 of the 1996-1 Pooling and Servicing
Agreement; (c) in the case of the Trust Estate of the 1997-1 transaction,
subsection (b)(xi) of Subsection 5.7 of the 1997-1 Sale and Servicing Agreement;
and (d) in the case of the Trust Estate of the 1998-1 Transaction, subsection
(b)(xi) of Section 5.7 of the 1998-1 Sale and Servicing Agreement; and (ii) all
funds otherwise distributable to NFAFT, if any, as Reversionary Holder (as
defined in the Master Spread Account Agreement), including funds distributed
pursuant to Section 3.03 thereof, or as holder of the Series 1997-1 Certificates
(as defined in the Master Spread Account Agreement) or as holder of the Series
1998-1 Certificates (as defined in the Master Spread Account Agreement), in each
case from the Spread Accounts maintained by Harris, as Trustee and Collateral
Agent, pursuant to the Master Spread Account Agreement (the amounts referenced
in the preceding clauses (i) and (ii), collectively, the "Free Cash Flow"), to
FUNB (together with its permitted successors and assigns, the "Lender") to
secure the payment of certain notes issued, and all other amounts payable,
pursuant to the Credit Agreement and subject to the Intercreditor Agreement
(including the Lender's rights to receive and compel payment under Section
1.2(e) thereof)(the "Debt"). Pursuant to this Letter and the Credit Agreement,
any and all Free Cash Flow paid to or distributable by you pursuant to the terms
of the Underlying Agreements or erroneously received by you shall promptly be
remitted to the Lender pursuant to the instructions below.

          2.   Instruction.

                    (a)  NAFI, in its individual capacity and as the sole
     certificateholder of NFAFT, and Keith Stein and Peter Sorensen, each a
     Co-trustee of NFAFT, each hereby irrevocably instructs the NFAFT Owner
     Trustee, to execute, as Trustee on behalf of NFAFT, the Certificate of an
     Officer of the Trust, the Supplement to Backup Servicing Agreement, by and
     among NAFI, NFAFT and Harris, the Credit Agreement, any related financing
     statement and any other documents related thereto, and hereby irrevocably
     instructs Harris, as trustee under the 95-1 Pooling and Servicing Agreement
     and the 96-1 Pooling and Servicing Agreement, and as Indenture Trustee
     under the 97-1 Indenture and the 98-1 Indenture, the 1997-1 Owner Trustee
     and the 1998-1 Owner Trustee of the trusts created by the Underlying
     Agreements, to immediately execute the applicable acknowledgment at the end
     of this letter, to deliver an original executed copy of the same to FUNB,
     and immediately upon receipt of written notice delivered to the Addressees
     set forth above from the Lender that NFAFT has borrowed funds pursuant


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<PAGE>   4


to the Credit Agreement, to make all payments and distributions at any time
constituting the Free Cash Flow to the Lender at the following account:

          Account Bank:            First Union National Bank
                                   301 South College Street/TW-10
                                   Charlotte, North Carolina 08288-0610

          Facsimile Number:        Fax: 704 374-3254
          Telephone Number:        Telephone: 704 374-4334
          Reference:               Contact: Hannah Carmody

          Account Party:           National Financial Auto Funding Trust -
                                   Trust Account for the benefit of
                                   First Union National Bank

          LNB ABA Number:          053000219

          Account Number:          2000003817829

               The Account described above is referred to herein as the
"Collection Account" and may be changed by the Lender upon five days prior
written notice to each of you.

                    (b)  Immediately upon receipt of written notice delivered to
     the addressees set forth above from the Lender that NFAFT has borrowed
     funds pursuant to the Credit Agreement, any Free Cash Flow received by you
     shall be held by you, as bailee (1) until the Lender has advised you in
     writing that no Debt remains due and owing to the Lender, for the sole
     benefit of the Lender and shall be paid to the Collection Account for the
     Lender as provided in Section A.2(1) above or as otherwise directed by the
     Lender, and (2) after notice from the Lender that no Debt remains due and
     owing to the Lender, for the sole benefit of NFAFT and NAFI and shall be
     paid to NFAFT and NAFI or as otherwise directed by NFAFT in writing. Upon
     execution of such Credit Agreement by the parties thereto and the
     hereinafter following acknowledgment by the NFAFT Owner Trustee, Harris,
     the 1997-1 Owner Trustee and the 1998-1 Owner Trustee and until the Lender
     has advised you in writing that no Debt remains due and owing to the
     Lender, (i) neither NFAFT, NAFI nor any affiliate of either of them shall
     have any right to receive any of the Free Cash Flow, and no portion thereof
     shall be delivered to NFAFT, NAFI, any affiliate of either or any other
     person except as provided herein and (ii) you shall not accept any
     instructions with respect to the payment or distribution of the Free Cash
     Flow from any person other than the Lender without the prior written
     authorization of the Lender. In addition to the notice requirements
     specified in the Underlying Agreements, you are hereby instructed to give
     concurrent written notice to the Lender, NFAFT Owner Trustee, 1997-1 Owner
     Trustee and 1998-1 Owner Trustee and Harris at the addresses designated
     below (x) of a copy of any notice given to or by you pursuant to the
     Underlying Agreements or (y) that you are exercising remedies upon the
     occurrence of either a default or an event of default under the Underlying
     Agreements, but any failure to give such notice shall not in any respect
     impede or impair your ability to exercise remedies under


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<PAGE>   5


     the Underlying Agreement, you shall have no liability resulting from your
     failure to give such notice, other than for your own gross negligence or
     willful misconduct and you shall be protected by the indemnification and
     reimbursement provided pursuant to the Underlying Agreements in exercising
     remedies under the Underlying Agreements notwithstanding any failure to
     give such notice, save and except for your own gross negligence or willful
     misconduct.

                    (c)  After execution of the following acknowledgment by you
     and the execution of the Credit Agreement by the parties thereto, neither
     NFAFT nor NAFI may take (unless the Lender shall have given its prior
     written consent) any of the following actions or instruct you to take any
     of the following actions, and you may not accept (unless the Lender shall
     have given its prior written consent) from NFAFT or NAFI any instructions
     relating to any of the following actions, until the Lender has advised you
     in writing that no Debt remains due and owing to the Lender:

               (1)  UNTIL the Insurance Termination Date (as defined in the
     Credit Agreement) related to the applicable Underlying Transaction (as
     defined in the Credit Agreement), any act by either NFAFT or NAFI to remove
     you as trustee under the Underlying Agreements unless such act or
     instruction is given either (a) by FSA (acting within the scope set forth
     in the Waiver and Consent attached hereto as Exhibit A) or (b) a person
     other than FSA, NFAFT or NAFI who is authorized to give such instruction
     pursuant to the Underlying Agreements; and thereafter, any act by any
     person to remove you as trustee under the Underlying Agreements and unless
     such act or instruction is given by a person who is (x) not FSA, NFAFT or
     NAFI and (y) otherwise authorized to give such instructions pursuant to the
     related Underlying Agreements.

               (2)  Until the Insurance Termination Date (as defined in the
     Credit Agreement) related to the applicable Underlying Transaction (as
     defined in the Credit Agreement), any act by either NFAFT or NAFI to amend
     either the NFAFT Trust Agreement or any other Underlying Agreement unless
     such act or instruction is given either (a) by FSA (acting within the scope
     set forth in the Waiver and Consent attached hereto as Exhibit A) or (b) a
     Person other than FSA, NFAFT or NAFI who is authorized to give such
     instruction pursuant to the Underlying Agreements; and thereafter, any act
     by any person to amend either the NFAFT Trust Agreement or any other
     Underlying Agreement unless such act or instruction is given by a person
     who is (x) not FSA, NFAFT or NAFI and (y) otherwise authorized to give such
     instructions pursuant to the related Underlying Agreements.

               (3)  Until the Insurance Termination Date (as defined in the
     Credit Agreement) related to the applicable Underlying Transaction (as
     defined in the Credit Agreement), any act by either NFAFT or NAFI to
     terminate the NFAFT Trust Agreement or Underlying Agreement unless such act
     or instruction is given either (a) by FSA (acting within the scope set
     forth in the Waiver and Consent attached hereto as Exhibit A) or (b) a
     Person other than FSA, NFAFT or NAFI who is authorized to give such
     instruction pursuant to the Underlying Agreements; and thereafter, any act
     by any


                                      -5-

<PAGE>   6


     person to terminate the NFAFT Trust Agreement or any other Underlying
     Agreement unless such act or instruction is given by a person who is (x)
     not FSA, NFAFT or NAFI and (y) otherwise authorized to give such
     instructions pursuant to the related Underlying Agreements.

                    Notwithstanding the foregoing provisions of this subsection
     2(c), Lender acknowledges that, until the Insurance Termination Date (as
     defined in the Credit Agreement), FSA may direct you to enter into
     amendments in accordance with the terms of the relevant Underlying
     Agreements but without the Lender's consent, to make changes to the
     Underlying Agreements that FSA believes in good faith and its reasonable
     business judgment are necessary, appropriate, or incidental solely to
     engage and retain a satisfactory successor servicer in the event of a
     servicer succession under the terms of the Underlying Agreements,
     including, without limitation, any amendment entered into in accordance
     with the terms of the relevant Underlying Agreements that make changes to
     either (a) the duties or standard of care of the servicer or (b) the
     compensation paid to the servicer even if the change in compensation paid
     to the servicer reduces or eliminates the amount of Free Cash Flow.

          3.   Free Cash Flow Held as Bailee. Immediately upon written notice
from the Lender that NFAFT has borrowed funds pursuant to the Credit Agreement
and until the Lender has advised you in writing that no Debt remains due and
owing to the Lender, NFAFT and NAFI hereby waive and renounce any and all rights
that they may have to the Free Cash Flow. Any Free Cash Flow received by you
shall be held by you, as bailee (1) until such time as the Lender has advised
you in writing that no Debt remains due and owing to the Lender, for the sole
benefit of the Lender and to be paid to the Collection Account for the benefit
of the Lender as provided in Section A.2(1) above or as otherwise directed by
the Lender, and (2) after written notice from the Lender that no Debt remains
due and owing to the Lender, for the sole benefit of NFAFT and NAFI and shall be
paid to NFAFT and NAFI or as otherwise directed by NFAFT in writing.

          4.   Covenants. By executing the acknowledgment below, you hereby
covenant and agree that you shall not institute any proceedings to adjudicate
any of NFAFT, 97-1 Owner Trust or 98-1 Owner Trust a bankrupt or insolvent,
consent to the institution of bankruptcy or insolvency proceedings against any
of NFAFT, 97-1 Owner Trust or 98-1 Owner Trust, file a petition seeking or
consenting to reorganization or relief under any applicable federal or state law
relating to bankruptcy, consent to the appointment of a receiver, liquidator,
assignee, trustee, sequestrator (or similar official) of any of NFAFT, 97-1
Owner Trust or 98-1 Owner Trust or any part of its property or admit its
inability to pay its debts generally as they become due or authorize any of the
foregoing to be done or taken on behalf of NFAFT, 97-1 Owner Trust or 98-1 Owner
Trust.

          5.   No Amendment. NAFI hereby covenants that no amendment of the
NFAFT Trust Agreement or any Underlying Agreement shall occur except upon the
prior written consent of the Lender.

          6.   Beneficiaries. The Lender and FSA shall be third party
beneficiaries of this Direction Letter.


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<PAGE>   7


          7.   Regarding Chase, Wilmington and Harris. Chase, Wilmington and
Harris shall be afforded all of the rights, protections, immunities and
indemnities provided in the Underlying Agreements as if the same were
specifically set forth herein, provided, however, that this section shall not be
deemed to give Chase, Wilmington and Harris any encumbrance or lien in respect
of the Free Cash Flow.

          8.   The security interest of the Lender in all Collateral other than
the First Priority Collateral (as defined in the Credit Agreement) is
subordinate to the interests of the Senior Lien Holder (as defined in the
Intercreditor Agreement), FSA and the interest of all noteholders and
certificateholders of the NAFI Trusts until the Insurance Termination Date with
respect to each such NAFI Trust and the Lender shall have no authority to direct
any Collateral Agent prior to such related Insurance Termination Date with
respect to the NAFI Trust to which such Collateral Agent's duties relate, other
than the authority to instruct that payments of Free Cash Flow, if any, be made
or the authority to give required consents as set forth in this letter.

          9.   In the event of a foreclosure by Harris (as Collateral Agent),
FSA, or any other person entitled to foreclose pursuant to the Underlying
Agreements or the Intercreditor Agreement, you shall dispose of or retain the
Subordinated Collateral (as defined in the Intercreditor Agreement) as directed
by such foreclosing creditor free and clear of the interests of the Lender
hereunder (unless the Lender is a person entitled to foreclose pursuant to the
Intercreditor Agreement and the Underlying Agreements).

          10.  Nothing contained herein is intended to or should be deemed to
restrict in any manner (i) the authority or discretion of the Controlling Party
under the Underlying Agreements or (ii) any of the rights or remedies available
to FSA or any other Controlling Party (as defined in the Underlying Agreements),
the Senior Lien Holder (as defined in the Intercreditor Agreement), or the
noteholders and certificateholders of the NAFI Trusts that have the benefit of
any Financial Guaranty Insurance Policy issued by FSA, under the Intercreditor
Agreement or the Underlying Agreements, other than, in either case, those
explicit references herein to the Waiver and Consent Letter to the extent an
applicable restriction is contained in the Waiver and Consent Letter.


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<PAGE>   8


                                     Sincerely,


                                     NATIONAL AUTO FINANCE COMPANY, INC.
                                     (f/k/a/ National Auto Finance Company L.P.)

                                     By:      /s/ KEITH B. STEIN
                                           -------------------------------------
                                     Name:        Keith B. Stein
                                           -------------------------------------
                                     Title:
                                           -------------------------------------

                                     Keith Stein, as Co-trustee of NFAFT

                                     /s/ KEITH B. STEIN
                                    --------------------------------------------
                                    Keith B. Stein

                                    Peter Sorensen, as Co-trustee of NFAFT

                                    /s/ PETER SORENSON
                                    --------------------------------------------
                                    Peter Sorenson


<PAGE>   9



     B.   ACKNOWLEDGEMENT FROM NFAFT OWNER TRUSTEE AND INSTRUCTIONS FROM NFAFT

          1.   Acknowledgment. The foregoing terms and provisions in Section A
above, including the irrevocable instructions from NFAFT, NAFI and the
Co-trustees, as well as the covenants and agreements, are hereby agreed to,
accepted and acknowledged.

          2.   Instruction to Harris, the 1997-1 Owner Trustee and the 1998-1
Owner Trustee (collectively, the "Underlying Trustees"). Each of the Underlying
Trustees are hereby instructed by NFAFT, immediately upon receipt of written
notice from the Lender that NFAFT has borrowed funds pursuant to the Credit
Agreement, to make all payments and distributions at any time constituting the
Free Cash Flow to the Collection Account in accordance with Section A above.

          3.   Notice of Default. The Underlying Trustees are hereby instructed
by NFAFT, to give written notice to the party designated below of either the
intended or actual exercise, if any, by Harris or the 1997-1 Owner Trustee or
the 1998-1 Owner Trustee, of remedies under the Underlying Agreements upon the
occurrence of a default or an event of default thereunder, but any failure to
give such notice shall not in any respect impede or impair your ability to
exercise remedies under the Underlying Agreements and you shall be protected by
the indemnification and reimbursement provided for in the Underlying Agreements
in exercising remedies under the Underlying Agreements notwithstanding any
failure to give such notice.

          4.   Subordination of Lender's Interests. The security interest of the
Lender in all Collateral other than the First Priority Collateral (as defined in
the Credit Agreement) is subordinate to the interests of the Senior Lien Holder
(as defined in the Intercreditor Agreement), FSA and the interest of all
noteholders and certificateholders of the NAFI until the Insurance Termination
Date with respect to each such NAFI Trust and the Lender shall have no authority
to direct any Collateral Agent prior to such Insurance Termination Date with
respect to the NAFI Trust to which such Collateral Agent's duties relate, other
than the authority to instruct that payments of Free Cash Flow, if any, be made
or the authority to give required consents to instructions as set forth in this
letter.

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Address of Lender:

First Union National Bank of North Carolina
301 S. College Street (TW-10)
Charlotte, North Carolina 28288-0610

Sincerely,

NATIONAL FINANCIAL AUTO FUNDING TRUST
By: Chase Manhattan Bank Delaware,
not in its individual capacity, but
solely as Owner Trustee of NFAFT

By: /s/ DENIS KELLY
    -----------------------------
Name:   DENIS KELLY
     ----------------------------
Title:  TRUST OFFICER
      ---------------------------

Dated: As of March 31, 1999



<PAGE>   11



     C.   HARRIS ACKNOWLEDGMENT

     The foregoing terms and provisions, are hereby agreed to, accepted and
acknowledged by Harris.

     The security interest of the Lender in all Collateral other than the Free
Cash Flow (as defined in the Credit Agreement) is subordinate to the interests
of the Senior Lien Holder (as defined in the Intercreditor Agreement), FSA and
the interest of all noteholders and certificateholders of the NAFI Trusts until
the Insurance Termination Date with respect to each such NAFI Trust and the
Lender shall have no authority to direct any Collateral Agent prior to such
Insurance Termination Date with respect to the NAFI Trust to which such
Collateral Agent's duties relate, other than the authority to instruct that
payments of Free Cash Flow, if any, be made or the authority to give required
consents to instructions as set forth in this letter.

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<PAGE>   12



HARRIS TRUST AND SAVINGS BANK,
an Illinois banking corporation,
not in its individual capacity, but
solely as Trustee of the National Auto Finance
1995-1 Trust and the National Auto Finance 1996-1 Trust,
as Indenture Trustee of the National Auto Finance 1997-1 Trust,
and the National Auto Finance 1998-1 Trust
and as Custodian and Collateral Agent of
the National Auto Finance 1995-1 Trust,
the National Auto Finance 1996-1 Trust,
the National Auto Finance 1997-1 Trust and
the National Auto Finance 1998-1 Trust

By:  /s/ E. KAY LEIDERMAN
   -----------------------------------
Name:    E. Kay Leiderman
     ---------------------------------
Title:   Vice President
      --------------------------------

Dated: March 31, 1999


<PAGE>   13

     WILMINGTON ACKNOWLEDGMENT

     The foregoing terms and provisions, including the irrevocable instructions
from NFAFT set forth in Sections B.1, B.2 and B.3 above are hereby agreed to,
accepted and acknowledged by 1997-1 Owner Trustee and the 1998-1 Owner Trustee.

     The security interest of the Lender in all Collateral other than the Free
Cash Flow (as defined in the Credit Agreement) is subordinate to the interests
of the Senior Lien Holder (as defined in the Intercreditor Agreement), FSA and
the interest of all noteholders and certificateholders of the NAFI Trusts until
the Insurance Termination Date with respect to each such NAFI Trust and the
Lender shall have no authority to direct any Collateral Agent prior to such
Insurance Termination Date with respect to the NAFI Trust to which such
Collateral Agent's duties relate, other than the authority to instruct that
payments of Free Cash Flow, if any, be made or the authority to give required
consents to instructions as set forth in this letter.

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<PAGE>   14


WILMINGTON TRUST COMPANY,
not in its individual capacity but solely as Owner Trustee
of the National Auto Finance 1997-1 Trust and
the National Auto Finance 1998-1 Trust,

By:   /s/ DENISE M. GERAN
    ----------------------------------------
Name: Denise M. Geran
     ---------------------------------------
Title: Senior Financial Services Officer
      --------------------------------------

Dated: As of March 31, 1999