<PAGE>

                                                                 EXHIBIT 10.18.1

            FIRST AMENDMENT TO THE WESTCORP EXECUTIVE DEFERRAL PLAN V
                             PARTICIPATION AGREEMENT

      THIS FIRST AMENDMENT TO THE WESTCORP EXECUTIVE DEFERRAL PLAN V
PARTICIPATOIN AGREEMENT, among WESTCORP, a California corporation, (hereinafter
referred to as the "the Plan Sponsor"), and the following subsidiaries: WESTERN
FINANCIAL ASSOCIATE SOLUTIONS, a California corporation, WESTERN FINANCIAL BANK,
a federally chartered savings bank, WFS FINANCIAL INC., a California
corporation, WestFin Insurance Agency, Inc., a California corporation, WFS
RECEIVABLES CORPORATION, a Nevada corporation, and WESTRAN SERVICES CORPORATION,
a California corporation (each individually referred to as "Subsidiary" and
collectively referred to as "Subsidiaries"), and the ADMINISTRATIVE COMMITTEE,
on behalf of the Westcorp Executive Deferral Plan V (the "Plan"), is effective
as of December 1, 2004.

                                    RECITALS

      WHEREAS, the parties have entered into a Westcorp Executive Deferral Plan
V Participation Agreement, effective as January 1, 2004. ("Agreement").

      WHEREAS, the parties desire to amend the Agreement to include WFS
Receivables Corporation 3, a California corporation, as a "Subsidiary" to the
Agreement;

      WHEREAS, WFS Receivables Corporation 3, and has been appoved as such by
the Administrative Committee pursuant to Article 2.1 of the Plan as an entity
eligible to sponsor the Plan; and

      WHEREAS, WFS Receivables Corporation 3, pursuant to resolutions adopted by
its Board of Directors, has accepted its designation as an entity eligible to
sponson the Plan, and has specifically indicated its desire to adopt the Plan
and be bound by its terms.

      NOW, THEREFORE, in consideration of the mutual covenants and promises
contained in the Agreement and in this Addendum and for other good and
sufficient consideration, the Agreement is amended as follows:

                                    AGREEMENT

      WFS Receivables Corporation 3 is included as a Subsidiary to the Agreement
effective as of December 1, 2004.

      Except as amended specifically herein, all terms and conditions of the
Agreement shall remain in full force and effect.

      All capitalized terms not herein defined shall have the same meaning as
set forth in the Agreement.

      This Amendment may be executed in any number of counterparts and by
separate parties hereto on separate counterparts, each of which when executed
shall be deemed an original, but all such counterparts taken together shall
constitute one and the same instrument.

                                        1
<PAGE>

      IN WITNESS WHEREOF, the undersigned have executed this Amendment on the
date set forth below.

WESTCORP

By: __________________________________                 Date:___________, 2004
    Thomas A. Wolfe, President

WESTERN FINANCIAL ASSOCIATE SOLUTIONS

By: __________________________________                 Date:___________, 2004
    Karen Marchak, President

WESTERN FINANCIAL BANK

By: __________________________________                 Date:___________, 2004
    Thomas A. Wolfe, President

WESTFIN INSURANCE AGENCY, INC.

By: __________________________________                 Date:___________, 2004
    Thomas A. Wolfe, President

WFS RECEIVABLES CORPORATION

By: __________________________________                 Date:___________, 2004
    John Coluccio, President

WESTRAN SERVICES CORPORATION

By: __________________________________                 Date:___________, 2004
    Shelley M. Chase, President

WESTCORP EXECUTIVE DEFERRAL PLAN V

By: __________________________________                 Date:___________, 2004
    Shaunna Monticelli on behalf of the
    Administrative Committee for the Plan

WFS RECEIVABLES CORPORATION 3

By: __________________________________                 Date:___________, 2004
    Keith Ford, Assistant Vice President