Read these cheats and you will leart how to unlock cars in this game:
Custom Compact: finish Custom Series Qualifier
Custom Coupe: get all gold medals at Split Second Grand Prix
Gangster car: finish Pursuit 3
Custom Muscle Car: finish Pursuit 6
Custom Sports car: get all gold medals at Speed Streak Grand Prix
Custom Roadster: get all gold medals at Point Of Impact Grand Prix
Custom SUV: finish Pursuit 4
Custom Pickup: finish Pursuit 5
Driver's Ed Car: finish driving 101 and get all gold medals
Police car: finish Pursuit 1
Classic car: finish Pursuit 2
Hot Rod car: finish Face Off 1
Stock car: finish Face Off #2