This cheat is easy to do and you can do as many times as you wish. Simply win a race day and choose your prize, then when it asks you if you want to continue to dominate turn the DS off. Turn it back on and go to your career and pick your prize again, and again, and etc.
1965 Pontiac GTO
Defeat Nate Denver (Speed King) in Nevada.
1971 Dodge Challenger R/T
Dominate the Challenge Race Day at Texas World Speedway.
BMW M3 E92
Defeat Ray Krieger (Grip King) at Willow Springs.
Coke zone car
Enter the code zerozerozero to unlock the coke zone car.
Ford Mustang GT
Defeat Karol Monroe (Drag King) at Infineon.
Hydraulics mini-game
Win a Raceday in Career mode that has the Hydraulics mini-game as an event.
Mazda RX-7
Defeat Aki Kimura (Drift King) at Ebisu.
Mitsubishi Lancer EVOLUTION
Defeat RYO (Showdown King) in Nevada.
Mitsubishi Lancer Evolution IX
Dominate the Challenge Race Day at Willow Springs.
Nissan 350Z
Dominate the Challenge Race Day on the Autobahn.
VW Golf GTI
Dominate the Challenge Race Day at Willow Springs (Battle Machine)