factory chrysler stereo questions

As Red fish said, most (all?) aftermarket amps can be run off of speaker inputs directly, or by using a speaker level to RCA adapter that will plug right into the amp. Any time you can add (well placed) speakers that are specific to certain frequencies, you will improve your sound stage/ over all sound. This is more true with the higher frequencies than lower/sub freqs. A good set of tweets wired in parallel, as 67Dart stated, will cost little and help expand the soundstage. In comparision, one good/ well powered sub is usually enough to fill a space with bass....although some will argue you need two. Make sure you add high pass filters to the tweets. It will protect them from the damageing lower freqs and give you a cleaner sound