When Infogrames logo appears on the white intro-screen, quickly press Up, Down, Triangle + Circle. The words "Lock Off" should appear. After that, enter any of the following codes to modify the game. Hold the final button (L1, R2, etc.) until you've selected your language, then release everything and get ready to roll.

Code               Effect
Left+L1           No time limit
Left+L2           18 extra narrow tracks
Left+R2           Lets you restart in Arcade mode
Left, Right       Debug mode (useless)
Left+L2+L2+R1+R2  All cheats

In the game progression screen in the options menu press:- L1, R1, LEFT, RIGHT, LEFT, RIGHT, UP, DOWN, UP, DOWN, X, X + SELECT. You will hear a beep if you did it correctly. From there highlight any empty box and press X to unlock that car or track.