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                                                                    Exhibit 10.4


                                     RELEASE

         For good and valuable consideration, as set forth in the Amendment to
Employment Agreement executed between the parties concurrently herewith, the
receipt and adequacy of which are hereby acknowledged and confessed, William C.
Saunders ("Saunders"), on behalf of himself and his heirs, successors, and
assigns (collectively, the "Releasing Parties"), hereby fully releases, acquits,
and forever discharges HighwayMaster Communications, Inc., formerly known as HM
Holding Corporation, a Delaware corporation, and HighwayMaster Corporation, a
Delaware corporation, and each of their respective predecessors, parents,
affiliates, and subsidiaries (each of the foregoing being called a
"HighwayMaster Company") and the respective employees, officers, directors,
agents, predecessors, successors, assigns, representatives, and all related
entities of each HighwayMaster Company (all of such persons being called
collectively, the "Released Parties"), of and from any and all actions, causes
of action, suits, debts, disputes, damages, claims, obligations, liabilities,
and demands of any kind whatsoever, at law or in equity, whether matured, or
unmatured, liquidated or unliquidated, vested or contingent, choate or inchoate,
known or unknown (collectively, "Claims") that the Releasing Parties (or any of
them) had, now has, or hereafter can, shall, or may have against the Released
Parties or any of them for, upon, or by reason of any matter, cause, or thing
whatsoever arising in connection with or related to Saunders being, on or before
the date of this Release, an officer, director, employee, or agent of, or having
any other relationship (contractual or otherwise) with, a HighwayMaster Company.
The released Claims also include but are not limited to claims arising under
Title VII of the Civil Rights Act of 1964, 42 U.S.C. ss. 2000e (relating to
employment discrimination), the Civil Rights Act of 1991, P.L. 102-166, the
Texas Human Relations Act, Tex. Rev. Civ. Stat. Art. 5221k or similar statutes
from other applicable states, the Age Discrimination in Employment Act, 29
U.S.C. ss. 621, and under any other state or federal statute or regulation, any
claims for breach of contract, tort, and personal injury of any sort, including
but not limited to those arising out of or relating in any way to Saunders'
employment by, association with and termination of employment from
HighwayMaster. This release covers claims both that Saunders knows about and
those that he may not know about, but does not include prospective rights which
Saunders has as a shareholder of the Company.

         Notwithstanding the foregoing, nothing in this Release is intended to
release any Claims arising on or after the date hereof under or pursuant to (a)
the provisions of that certain Employment Agreement dated as of February 4,
1994, executed by HighwayMaster Corporation and Saunders, as amended through
September 1, 1998 (except to the extent such September 1, 1998 amendment of the
Employment Agreement releases, by its own terms, certain claims and provisions
contained in the Employment Agreement prior to its amendment) that are
applicable after or arise as a result of termination of employment, including
but not limited to the right to obtain the payment of $300,000 as provided in
Section 7(b) thereof on January 21, 1999 and the rights under Section 21
thereof; (b) that certain Stock Option Agreement executed by HighwayMaster
Communications, Inc. and Saunders dated as of June 13, 1994, as amended on
September 18, 1998, effective as of June 13, 1994; (c) that certain Stock Option
Agreement executed by HighwayMaster Communications, Inc. and Saunders dated as
of September 18, 1998; or (d) any other claims that arise from matters that
are included in clauses (a) through (e) of that certain acknowledgment dated as
of even date herewith executed by Saunders.


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         Saunders hereby agrees that he will not assert, and that he is estopped
from asserting, against the Released Parties, or any of them, any Claim that he
has released in this Release.

         This Release will be null and void and of no force or effect as to any
Released Party with respect to any Releasing Party if that certain Release dated
the date hereof executed by Highwaymaster Communications, Inc., formerly known
as HM Holding Corporation, a Delaware corporation, and HighwayMaster
Corporation, a Delaware corporation, for the benefit of any Releasing Party is
found, in a final, nonappealable order of a court of competent jurisdiction
entered after pursuit and exhaustion of all appeals, to be ineffective with
respect to such Releasing Party in some material respect and such Released Party
or its successor in interest asserts a Claim against such Releasing Party that
would have been released but for such order.

         No waiver or amendment of this Release, or the promises, obligations,
or conditions herein, shall be valid unless set forth in writing and signed by
the party against whom such waiver or amendment is to be enforced, and no
evidence of any waiver or amendment of this Release shall be ordered or received
in evidence in any proceeding, arbitration or litigation between the Releasing
Parties (or any of them) and the Released Parties (or any of them) arising out
of or affecting this Release unless such waiver or amendment is in writing and
signed as stated above.

         Saunders hereby represents and warrants that he has not assigned,
pledged, or transferred in any manner to any person or entity any Claim that is
the subject of this Release. Saunders shall indemnify the Released Parties and
each of them from and against all Claims that are the subject of this Release
that are asserted by any person or entity by or through any Releasing Party or
as a result of any assignment, pledge, or transfer that caused the foregoing
representation to be false.

         The provisions of this Release shall be specifically enforceable.

         Executed in one or more counterparts as of December 15 , 1998.

                                               /S/ WILLIAM C.  SAUNDERS   
                                               --------------------------------
                                                   William C. Saunders




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