When the Infogramme Logo appears press Up, Down, Triangle, Circle, Up, Down, Triangle and Circle. The word "lockoff" will appear. Once you have done this, input the following codes:
To get the Toyota Celica, complete the Arcade and Championship Modes on Ace level. Race Corsica on Time Attack Mode and get a lap time of under one minute. You should now see that the barrier that was on the left is gone, revealing a road. Take that road to collect the Celica.