These cars in the Challenge Series are unlocked by completing the indicated task.

Unlock Dodge Charger RT Classic (Muscle):
Defeat the 21st Muscle Car Gang.

Unlock Jaguar XK 2007 (Exotic):
Complete ALL 3 Turf War races.

Unlock Mazda RX7 (Tuner):
In Boss Battle mode defeat Kenji.

Unlock Aston Martin DB9 (Exotic):
In Boss Battle mode defeat Wolf

Unlock Nissan 240SX (Tuner):
Complete ALL 3 Checkpoint challenges.