Hold L+R and press up left down right left right left right.
Change name to "xopensez" and you will get everything unlocked.
Unlock All Cars And Courses
At the Main Menu, select Options, and on the Game Options screen, select Change
Name, and enter XOPENSEZ (case-sensitive) as your player name. If you entered the
code correctly, all cars and courses will be available.
To unlock everything type "igiveup" as your name.