Get to the code screen by selecting "Options" from the Main Menu, and then going to the "Bonus" screen. At the Bonus screen choose the :Enter Cheat Code" option and then enter the following cheats from the list below.
As you enter cheats, you should see them listed on the Activate Bonuses screen. Note: The codes don't work in Championship Mode.
To unlock cars, you have to collect a minimum number of points in Championship Mode, according to the table below: