At the start screen with the BMW car when you see it type in the code:
iammostwanted to unlock all cars in carrer mode.

(this include porsche 911 turbo s,Aston martin DB9 ect.)

If the following code doesn't work then try this:
Iammostwanted

Enter one of these codes at the Main Menu:

Iammostwanted - unlocks all cars
Givemethegto - releases the GTO
Castrol - unlocks a Ford GT with Castrol decals
Burgerking - unlocks Burgerking Challenge in the challenge series