These cars are unlocked when you beat the specified racer in a Star Showdown.

Car	How to unlock
Dodge Challenger SRT8 392 (A Class)	Beat Ali Howard.
Ferrari 599XX	Beat Darius Flynt.
Ford F150 SVT Raptor (B Class)	Beat Ramona Cravache.
Lamborghini Diablo SV (A Class)	Beat Duke Maguire.
Lexus LF-A (S Class)	Beat Zaki Malik.
Nissan GT-R Black Edition (S Class)	Beat Marko Bavan.
Saleen S7 (R3 Class)	Beat Hailey Harper.