When you are on the main menu scroll to the options menu and when you are on options find cheat codes and press space bar and then type whatever cheat code you want and click spacebar to activate it

rimbuk - All cities, vehicles and modes in arcade mode

starpower - F to fire gun and disables damage

leatherandlace - F to fire gun, H to fire rocket, and disables damage

howhardcanitbe[0-9] - Set difficulty (0=easiest, 9=hardest)

howfastcanitbe[0-9] - Set speed of other cars (0=slow, 9=fast)

starlite - Trippy Mode (Psychedelic View)

yerbamate - Unlimited nitrous

theparrot - Unlock all racing modes

griswold - Unlock all cities

octane - Nitrous

tumbler - Allows Cars to do Wheelies in air.

upupdowndown - Turns off all cheats