Unlockable: 1969 Dodge Charger and Subaru Impreza WRX STi
Select the car and proper race level until you're goiing up against the boss car you want to unlock. Then beat the car at the PRO-level Boss Races. You'll find them available in your pocket garage.

Unlockable: Chevrolet Corvette Z06 and Nissan Skyline R34 GTR
Select the car and proper race level until you're goiing up against the boss car you want to unlock. Then beat the car at the MASTER-level Boss Races. You'll find them available in your pocket garage.