At the main menu (the living room), go to the options screen, and then enter the codes... - A horn will confirm correct code entry.
Invincibility:
Hold down L + R, then press Y, A, Y, A.
Different Camera:
Hold down L+R, then press B, B, B, A.
Fast Cars:
Hold down L+R, then press X, X, X, X.
Faster Cars:
Hold down L+R, then press Y, Y, Y, Y.
Grid View:
Hold down L+R, then press B, A, B, Y.
Pressing Horn Button Jumps:
Hold down L+R, then press X, X, X, Y.
View Credits:
Hold down L+R, then press A, X, X, Y.
Show Speedometer:
Hold down L+R, then press Y, Y, B, X.
Trippy View:
Hold down L+R, then press Y, B, Y, B.
One-Hit Wrecks:
Hold down L+R, then press Y, Y, X, X.
Unlock Red Brick Car:
Hold down L+R, then press B, B, Y, X.
Beat the Clock:
During timed missions the clock will not start until you get into your car. To get a head start simply run to the area you need to get to and then steal a car or visit a phone booth to get a car and continue the mission.
Unlock Itchy and Scratchy Cartoon:
Collect all the cards in every level of the game to unlock an episode of Itchy & Scratchy
Unlock Multiplayer Racing Game:
Collect all the cards in just one level to unlock the multi-player racing mini game.
Holiday Look:
Change your system's internal clock to the dates for Thanksgiving, Halloween and Christmas to experience a different game look.