Pause the game and highlight the "Map" option. Then, press Triangle, X(2), Triangle, Square, Circle(2), Square, X, Square, Circle, Triangle. If you entered the code correctly, you will hear the sound of an engine. 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.