Select "Driver DNA Lab", then choose "Celebrity Face Off". Defeat the indicated driver in all three of his challenges to unlock the corresponding car.

Car	How to unlock
Audi 1.8T Quattro	Caballero.
Dodge Viper SRT-10	Samuel Hubinette.
Ford Mustang Fastback	Djibril Cisse.
Ford Mustang GT '05	Smudo.
Nissan Silvia S15 Spec-R	Phil Morrison.
Nissan Skyline R32 GTS-t Type M	Tarzan Yamada.
Toyota Corolla GT-S	Beau Yates.