Cheat Mode:
Go to the preferences menu and press L1 + R1.  Then enter the following codes at the trick screen.  Press X to enable.

SLOW MO ON  Runs game at half speed.
LO GRAV ON  Reduces gravity.
RAISE THEM  Turns player invincible.
WATTO SHOP  All Characters.
FIRERATEUP  Increase firing rate.
THROTTLEUP  For high speed.
BFM FEELIT  For heavy vehicles.
MULTI CARS  Select the same vehicles in 2 player mode.
Movie Show  Show all victory movies
No Baddies  Battle mode opponents can be set to none
Sad Movies  Show all the 'lose' movies
Extra Butts Altered controller
No Peaking  Random races and opponents

All Vehicles:
Go to prefrence in the game menu and press R1 + L1 (at the same time) then it should bring up jedi mind trick put in watto_shop.

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Hints

Darth Maul:
Beat the game on tournament mode with the rancor to unlock the cloud car, Princess Leia, and Darth Maul.

Thanks to Revolution reader ColdSlither, Tony Lindemann, Witten, Nick Mozes, Billy Goodman, Andrew Woodiwiss and Brian O'Banion!