Head for the in-game cheats menu to unlock the following effects:
PING - Unlocks all cars (Arcade Mode). If that hasn't worked then try putting in PINT instead and hopefully that will unlock all the cars in arcade mode (courtesy leosdemise).

Finish the game with 100% respect from all computer players. You must have 1,500 respect points from all of them to accomplish this.

You must acquire 1,500 respect points from all CPU characters.