This Cheat will give you the secret car.
First, you must become champion of the World Grand Prix. You 
can use any difficulty setting or the damage option. Save the results 
to your Controller Pak. When you restart the game, press and hold 
down the A and Buttons while on the title screen. Wait until the 
Game Mode Select screen appears. When you select any game 
mode, Team Forti will have a new F1 car.