At the main menu, select Options and then select Credits. At the credits screen, enter the Following codes to unlock the corresponding Cheats. If you entered the cheat correctly, You'll hear a confirming tone. (Note:These codes Will only work when you play in the single or Multiplayer races. Not in the Career Mode.)
Unlock Set #1: down, left, down, right, up, up. Unlock Set #2: left, left, down, down, right, right. Unlock Set #3: down, down, left, left, up, down. Unlock Set #4: right, down, right, down, left, up. Unlock Set #5: left, left, right, right, down, left. Unlock Set #6: right, up, up, up, down, left. Unlock Set #7: left, left, left, up, up, right. Unlock Set #8: right, up, left, up, down, right. Unlock Set #9: down, up, down, left, left, down. Unlock Set #10: up, up, down, down, left, right.
Use car from different class: To be able to use a car from one class while racing in another class you have to start your game with the game course type that you want to race in.
Then go to the Game Settings screen and select the class of car you want to use. Then press B to exit back to the lobby and go to Car Selection screen and you will then be able to choose a different class of car from the race you are in.
This can only be done online and you must be the host of the game.