Beat the missions listed to get the corresponding prize:

Super Buggy: Available at beginning of the game
Super Buggy Boost: Mission 3
D-5 Hondo: Training Mission 3
D-5 Hondo Oil Slick: Mission 8
Baja Truck: Mission 4
Baja Truck Bombs: Mission 7
ATV Monster: Mission 10
ATV Monster s Speed Burst: Mission 14
Du Monde: Mission 13
Du Monde Oil Slick: Mission 21
Sahara Special: Mission 17
Sahara Smoke Screen: Mission 18
Grenadier: Mission 24
Grenadier Bombs: Mission 25
Kavostov Halftrack: Mission 27
Kavostov Halftracks Smoke Screen: Mission 29
Vietnam Map: Mission 9
Georgian Russia Winter Map: Mission 22