Being that you still have the factory HU, the best thing you could do is to scrap that, and buy yourself a good quality HU; Alpine, Clarion, Eclipse, Nakamichi. If you were to simply add an amp to your factory HU, it's still going to sound like %#it. I'm not sure if your car has the premium stereo in which case there is allready an amp behind your glovebox which is the most god awful thing ford ever created, and that needs to be bypassed. Goodluck and welcome aboard.
