To get these unlockables, do the following:
2 Player Pursuit Mode: beat Pursuit 3
Police car: complete Pursuit 1
Classic car: complete Pursuit 2
Gangster car: complete Pursuit 3
Custom SUV: complete Pursuit 4
Custom Pickup: complete Pursuit 5
Custom Muscle Car: complete Pursuit 6
Custom Compact: complete Custom Series Qualifier
Custom Coupe: get all gold medals at Split Second Grand Prix
Custom Sports car: get all gold medals at Speed Streak Grand Prix
Custom Roadster: get all gold medals at Point Of Impact Grand Prix
Hot Rod car: complete Face Off 1
Stock car: complete Face Off 2