When in the garage type

 flashback         One day back
 moneycheat        $1000

 After typing in the cheat click on the light bulb icon and the desired effect
 will take place.

 ---

 1. Open your hex editor and go into the file below (or wherever you have installed the game):
    C:\Program Files\Activision Value\Monster Garage\save\career
 2. Look for NAME-# and open up that directory.
 3. Look for the file 'main' no extension.
 4. Open that file up and go to OFFSET 00020 and change the set off four to FFFF
 5. Save the file.

 If successful you will now have $65000 when you load and start the game.

 Collected with the help of

 Samurai, Samurai_@web.de