If you have at least an A level license and want to unlock cars with company skins on them, insert the following passwords at the in-game cheats menu.
Best Buy Car - BESTBUY
Circuit City Car - CIRCUITCITY
Gamestop Car - GAMESTOP
Wal-Mart Car - WALMART
Unlockable: Steel Wheels Car
To unlock the Steel Wheels car, head to the in-game cheats menu, where you can insert "Sponsor Codes". Insert the following password to unlock the car: U84D 3Y8K FY8Y 58N6.
Submitted by TigerX7
I got this from IGN guys. enjoy. you need to have a certain license level to unlock these.