Roadkill Cheats

You can get all the gang cars in a level and some other cool cars if
you do the side missions

Find a short hill, then hold Down and jump it.

During gameplay, pause your game and highlight the map option. Next enter the following combo:
Triangle, X(2), Triangle, Square, Circle(2), Square, X, Square, Circle, Triangle.

After pausing your game, by pushing Start, highlight the MAP option. Now push Triangle, X(2), Triangle, Square, Circle(2), Square, X, Square, Circle, Triangle. You will hear the sound of an engine if you have entered the code correctly. You still need to find where the original weapon power-up appears, then collect it again. It will also cause some weapons which you have not unlocked to appear next to your garage. However, not all weapon power-ups are next to your garage. They are usually nearby, but on a ledge or around the block.

When you progress enough, you will be taken to another city by following the "M" on the map. You can actually go back to the previous city at anytime. Pay attention where the exits are for the cities.

Press START then type in Triangle, X, X, Triangle, Square, Circle(2), Square, X, Square, Circle, Triangle. After which the sound of an "ENGINE" will confirm correct code.