Various cheats
     Enter  the  following  codes  as  a  profile name to activate the
indicated  cheat  effect. After entered the codes, you may rename your
profile to any desired name.

or

     Go  to  the main menu, select options and then go to "Cheats" and
enter there.

Unlock School bus       Input "GIEVCARPLZ" as a profile name.
Unlock Rocket           Input "KALJAKOPPA" as a profile name.
Unlock Pimpster         Input "RUTTO" or "BIGTRUCK" as a profile name.
Unlock Flatmobile       Input "WOTKINS" as a profile name.
Unlock El Diablo        Input "ELPUEBLO" as a profile name.
Unlock Ice cream truck  Input "RAIDERS" as a profile name.
Get 1 million credits   Input "GIVECASH" as a profile name.
Get 1 million and unlock all cars   Input "GIEVEPIX" as a profile name. 
All tracks unlocked     Input "GIVEALL" as a profile name.

Unlockable cars:
Unlock Bullet GT - Win 1st place in all cups and stunts in Career mode. 
Unlock Insetta - Win Race Finals class cup in the Grand Finals
Unlock Venom - Win Derby Finals class cup in the Grand Finals