If your going to keep your factory deck, the esiest way to do it is to tap the back speakers and get their signal. You need a Line-out converter which basically takes speaker signal and converts it into RCA signal which then can be connected to an amp. The sound quality isn't quite as good as if you were to go from the back of an aftermarket deck, but to be honnest for people that aren't in sound competitions, you wouldn't know the difference. My system has been in my car for YEARS and I never had a problem. Just make sure you get your phasing right.
