Cheat List:
Enter these codes at the main menu. A sound indicates that the cheat is activated and the cheat menu in the options becomes available.
All Weapons: L, L, X, Y, Y, R, R
Invincibility: X, Y, L, R, L, R, R
Unlimited Ammo Cheat Unlocked: R, R, L, L, X, Y, Y
Unlock All Missions: X, X, Y, Y, R, R, L
Unlock All Vehicles: X, X, Y, Y, L, R, L
Immunity: X, Y, R, R, L, L, Y
Unlockable: GT40:
Go to take a ride. You can do what ever you want just make sure its in Miami. Also make sure that its the first starting point. When its done loading take the first left. Then take another left and follow the highway down until there?s a right turn that heads down into a dead end. At the end of the street go into the garage at the very end and walk into it (it should open) and there is a GT40.