These codes are entered at the 'Enter Code' menu which is found by going to 'Options' and then 'Career Mode'. Some of these codes can only be used once so it is advised that you disable the 'Auto-Save' option so you can revert the game back to it's original setting.

Unlock Audi TT 3.8 Quattro:
Enter the code 'ITSABOUTYOU' to unlock the Audi TT 3.8 Quattro in Career mode.