Unlock the following cars by entering the corresponding sequences of buttons at "Main Menu" 
then "Options". The unlocked car will become available in the garage in Quick Play mode: 

Car                    Code
-------------------------------------------
AMX Coyote           - X, LEFT, X, LEFT, X, LEFT.
Computer Spiele car  - X, UP, X, UP, X, UP.
Disney XD car        - X, DOWN, X, DOWN, X, DOWN.