<PAGE>
                                                                   EXHIBIT 4.1.8
 
                               AMENDMENT NO. 6
                                     TO
                        SECURITIES PURCHASE AGREEMENT

          This Amendment No. 6 to Securities Purchase Agreement (this
"Amendment") dated as of December 31, 1998 is entered into by and among each of
 ---------                                                                     
the parties to the Securities Purchase Agreement dated as of July 11, 1997 (the
"Securities Purchase Agreement") by and among FirstAmerica Automotive, Inc., a
 -----------------------------                                                
Delaware corporation (the "Company"), the Guarantors (as defined therein) and
                           -------                                           
the purchasers listed on the signature pages thereto, as amended.

          WHEREAS, the parties hereto desire to amend certain provisions of the
Securities Purchase Agreement.

          NOW, THEREFORE, in consideration of the foregoing, and other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties hereto agree as follows:

     Section 1. Definitions.  For all purposes of this Amendment:
                -----------                                      

          (a) Capitalized terms used but not defined herein shall have the
     respective meanings assigned to such terms in the Securities Purchase
     Agreement; and

          (b) The terms "hereby," "hereto," "hereof" and "herewith" and other
     words of similar import refer to this Amendment.

     Section 2. Minimum Consolidated Interest Expense Coverage Ratio.  Section
                ----------------------------------------------------          
5.12 of the Securities Purchase Agreement is hereby amended and restated in its
entirety to read as follows:

        5.12 Minimum Consolidated Interest Expense Coverage Ratio.
             ---------------------------------------------------- 

             The Company shall not permit its Consolidated Interest Expense
        Coverage Ratio for the four fiscal quarters ending on or about the
        date listed below, to be less than the correlative levels for such
        dates shown below; provided, that for the September 30, 1997, December
        31, 1997 and March 31, 1998 Measurement Dates, the Consolidated
        Interest Expense Coverage Ratio shall be calculated for the one, two,
        and three fiscal quarters, respectively, ending on such dates.

<TABLE> 
<CAPTION> 

        Measurement Date                       Minimum Ratio
        ----------------                       -------------
        <S>                                    <C> 
        September 30, 1997                               2.0
        December 31, 1997                                2.0
        March 31, 1998                                   2.1
        June 30, 1998                                    2.1
        September 30, 1998                               2.2
        December 31, 1998                                1.9
        March 31, 1999                                   1.9
        June 30, 1999                                    2.3
        September 30, 1999                               2.4
        December 31, 1999                                2.4
        March 31, 2000                                   2.5
        June 30, 2000 and thereafter                     2.6
</TABLE>
<PAGE>
 
     Section 3. Definition of Consolidated Interest Expense Coverage Ratio.  The
                ----------------------------------------------------------      
definition of the term "Consolidated Interest Expense Coverage Ratio" appearing
in Section 9.1 of the Securities Purchase Agreement is hereby amended by
inserting the following provision at the end of such definition:

             Notwithstanding any provision hereof to the contrary, for the
         sole purpose of calculating the Company's Consolidated Interest
         Expense Coverage Ratio for the year ended December 31, 1998, the term
         "Capital Expenditures" shall not include the aggregate amount of
         capital expenditures that is both (i) made by the Company for
         leasehold improvements to its corporate headquarters located at 601
         Brannan Street, San Francisco, California and (ii) fully reimbursed
         to the Company in cash by Thomas A. Price on or prior to January 31,
         1999.
         
     Section 4. Miscellaneous.
                ------------- 

          (a) THIS AMENDMENT AND ALL ISSUES HEREUNDER SHALL BE GOVERNED BY AND
     CONSTRUED IN ACCORDANCE WITH THE INTERNAL LAWS OF THE STATE OF CALIFORNIA
     (WITHOUT REFERENCE TO PRINCIPLES OF CONFLICTS OF LAW).

          (b) Upon the execution and delivery of this Amendment, the Securities
     Purchase Agreement shall be amended in accordance herewith and this
     Amendment shall form a part of the Securities Purchase Agreement for all
     purposes, and the parties hereto and every Holder shall be bound by the
     Securities Purchase Agreement, as so amended.

          (c) This Amendment may be executed in as many counterparts as may be
     deemed necessary and convenient, and by the different parties hereto on
     separate counterparts each of which, when so executed, shall be deemed an
     original, but all such counterparts shall constitute one and the same
     instrument.

          (d) The Section headings of this Amendment are for convenience of
     reference only and shall not be deemed to modify, explain, restrict, alter
     or affect the meaning or interpretation of any provision hereof.

                              [Signatures Follow]

                                       2
<PAGE>
 
          IN WITNESS WHEREOF, this Amendment has been duly executed by the
parties set forth below as of the date first written above.

                                        Company:
                                        ------- 

                                        FIRSTAMERICA AUTOMOTIVE, INC.



                                        By: /s/Debra Smithart
                                           -----------------------------
                                        Name: Debra Smithart
                                             ---------------------------
                                        Title: Chief Financial Officer
                                              --------------------------

                                       3
<PAGE>
 
Guarantors:
--------------------------------------
 
FAA SAN BRUNO, INC.                     FAA STEVENS CREEK, INC.
 
 
By: /s/ Thomas A. Price                 By: /s/ Thomas A. Price
   ---------------------------             -----------------------------
Name: Thomas A. Price                   Name: Thomas A. Price
     -------------------------               ---------------------------
Title: President                        Title: President
      ------------------------                --------------------------

SMART NISSAN, INC.                      FAA DEALER SERVICES, INC.

 
By: /s/ Thomas A. Price                 By: /s/ Thomas A. Price
   ---------------------------             -----------------------------
Name: Thomas A. Price                   Name: Thomas A. Price
     -------------------------               ---------------------------
Title: President                        Title: President
      ------------------------                --------------------------

TRANSCAR LEASING, INC.                  FAA CONCORD H, INC.
 
 
By: /s/ Thomas A. Price                 By: /s/ Thomas A. Price
   ---------------------------             -----------------------------
Name: Thomas A. Price                   Name: Thomas A. Price
     -------------------------               ---------------------------
Title: President                        Title: Vice-President
      ------------------------                --------------------------

FAA CONCORD N, INC.                     FAA CONCORD T, INC.
 
 
By: /s/ Thomas A. Price                 By: /s/ Thomas A. Price
   ---------------------------             -----------------------------
Name: Thomas A. Price                   Name: Thomas A. Price
     -------------------------               ---------------------------
Title: President                        Title: President
      ------------------------                --------------------------

FAA POWAY D, INC.                       FAA POWAY T, INC.
 
 
By: /s/ Thomas A. Price                 By: /s/ Thomas A. Price
   ---------------------------             -----------------------------
Name: Thomas A. Price                   Name: Thomas A. Price
     -------------------------               ---------------------------
Title: President                        Title: President
      ------------------------                --------------------------

FAA POWAY H, INC.                       FAA DUBLIN VWD, INC.
 

By: /s/ Thomas A. Price                 By: /s/ Thomas A. Price
   ---------------------------             -----------------------------
Name: Thomas A. Price                   Name: Thomas A. Price
     -------------------------               ---------------------------
Title: Vice-President                   Title: President
      ------------------------                --------------------------

FAA DUBLIN N, INC.                      FAA SERRAMONTE H, INC.
 
 
By: /s/ Thomas A. Price                 By: /s/ Thomas A. Price
   ---------------------------             -----------------------------
Name: Thomas A. Price                   Name: Thomas A. Price
     -------------------------               ---------------------------
Title: President                        Title: Vice-President
      ------------------------                --------------------------

                                       4
<PAGE>
 
FAA SERRAMONTE L, INC.                  FAA SERRAMONTE, INC.
 
 
By: /s/ Thomas A. Price                 By: /s/ Thomas A. Price
   ---------------------------             -----------------------------
Name: Thomas A. Price                   Name: Thomas A. Price
     -------------------------               ---------------------------
Title: President                        Title: President
      ------------------------                --------------------------

FAA BEVERLY HILLS, INC.                 FAA AUTO FACTORY, INC.
 
 
By: /s/ Thomas A. Price                 By: /s/ Thomas A. Price
   ---------------------------             -----------------------------
Name: Thomas A. Price                   Name: Thomas A. Price
     -------------------------               ---------------------------
Title: President                        Title: President
      ------------------------                --------------------------

FAA CAPITOL N, INC.                     DSW & ASSOCIATES, INC.
 
 
By: /s/ Thomas A. Price                 By: /s/ Thomas A. Price
   ---------------------------             -----------------------------
Name: Thomas A. Price                   Name: Thomas A. Price
     -------------------------               ---------------------------
Title: President                        Title: President
      ------------------------                --------------------------


FAA WOODLAND HILLS VW, INC.             DEALERSOFT  TECHNOLOGIES CORPORATION
 
 
By: /s/ Thomas A. Price                 By: /s/ Thomas A. Price
   ---------------------------             -----------------------------
Name: Thomas A. Price                   Name: Thomas A. Price
     -------------------------               ---------------------------
Title: President                        Title: President
      ------------------------                --------------------------


                                       5
<PAGE>
 
Holders
-------

TCW/CRESCENT MEZZANINE PARTNERS, L.P.
TCW/CRESCENT MEZZANINE TRUST
TCW/CRESCENT MEZZANINE INVESTMENT PARTNERS, L.P.

By:  TCW/CRESCENT MEZZANINE, L.L.C.,
     its general partner or managing owner


By: /s/ Jean-Marc Chapus
   ------------------------------
Name: Jean-Marc Chapus
     ----------------------------
Title: President
      ---------------------------


TCW LEVERAGED INCOME TRUST, L.P.

By:  TCW ADVISORS (BERMUDA), LIMITED,
     as General Partner


By: /s/ Mark L. Attanasio
   ------------------------------
Name: Mark L. Attanasio
     ----------------------------
Title: Group Managing Director
      ---------------------------

By:  TCW INVESTMENT MANAGEMENT COMPANY,
     as Investment Advisor


By: /s/ Jean-Marc Chapus
   ------------------------------
Name: Jean-Marc Chapus
     ----------------------------
Title: Managing Director
      ---------------------------

                                       6
<PAGE>
 
TCW LEVERAGED INCOME TRUST II, L.P.

By:  TCW (LINC II), L.P.,
     as General Partner

     By:  TCW ADVISORS (BERMUDA), LIMITED,
          as General Partner


          By: /s/ Mark L. Attanasio
             ------------------------------
          Name: Mark L. Attanasio
               ----------------------------
          Title: Group Managing Director
                ---------------------------

By:  TCW INVESTMENT MANAGEMENT COMPANY,
     as Investment Advisor


By: /s/ Jean-Marc Chapus
   ------------------------------
Name: Jean-Marc Chapus
     ----------------------------
Title: Managing Director
      ---------------------------

CRESCENT/MACH I PARTNERS, L.P.

By:  TCW ASSET MANAGEMENT COMPANY,
     as investment manager and attorney-in-fact


By: /s/ Jean-Marc Chapus
   ------------------------------
Name: Jean-Marc Chapus
     ----------------------------
Title: Managing Director
      ---------------------------

By: /s/ Mark L. Attansio
   ------------------------------
Name: Mark L. Attansio
     ----------------------------
Title: Group Managing Director
      ---------------------------


TCW SHARED OPPORTUNITY FUND II, L.P.

By:  TCW INVESTMENT MANAGEMENT COMPANY,
     its investment advisor


By: /s/ Jean-Marc Chapus
   ------------------------------
Name: Jean-Marc Chapus
     ----------------------------
Title: Managing Director
      ---------------------------

By: /s/ Mark L. Attansio
   ------------------------------
Name: Mark L. Attansio
     ----------------------------
Title: Group Managing Director
      ---------------------------

                                       7