From the main GRID menu go to Options > Bonus Codes > Enter Code and enter one of the following.
||Aston Martin DBR9 play.com Livery
||BMW 320si Buchbinder Livery
||eBay Motors Mustang
||Pagani Zonda Micromania Livery
||Unlocks all stage#1 muscle cars
||Unlocks all Tuner cars
||WTCC Spec BMW 320si Gamestation Livery
Get every trophy and complete every head-to-head race in Grid World to unlock Ninja Difficulty.
Unlock all Cheats
Complete all the global events in 1st place to unlock the final Head 2 Head with Ravenwest. If you win this final race you will unlock all the in game cheats. Once unlocked you can activate them by goin to Main Menu> Options > Bonus Codes. Once there you can select the cheats you wish to activate.