https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=292353394

If you want to play with vehicles from ProjectMarica:

Open Postal2CoOp.ini
...\steamapps\common\POSTAL2Complete\WorkshopContent\737631813\Postal2CoOp.ini

find there [Engine.GameEngine]

and input this

ServerPackages=ProjectMarica
ServerPackages=ProjectMaricaCore
ServerPackages=MaricaSpaceShip
ServerPackages=MaricaSnowMobile
ServerPackages=MaricaSilverDragonCars
ServerPackages=MaricaSegway
ServerPackages=MaricaSanchez
ServerPackages=MaricaRigCar
ServerPackages=MaricaQuad
ServerPackages=MaricaDragon
ServerPackages=MaricaCinemaTools
ServerPackages=Gallardo

You will find vehicles on custom levels, such as
-cus-ProjectMaricaCity
-cus-ProjectMaricaOasis
-cus-SeasideHill
-mpDGT-CrashArena
-mpDGT-HighwayStar[PM]
-mpDGT-Island
-mpDGT-ProjectMaricaCity
-mpDGT-ProjectMaricaOasis
-mpDGT-SeasideHill

P.S. If you haven't tried to play with Project Marica before, start it in single player workshop first to change Enter/Exit key bindings!