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 Coke Zero Volkswagen Golf GTI:
Enter the code 'ZEROZEROZERO' to unlock the Coke Zero Volkswagen Golf GTI In Career mode.