instead what most car makers do is put in a sound processor which takes the signals for the speakers and converts it to a usable signal(mid, treb, bass) that can be used by an amp
some amps can be hooked up using just the speaker signal wires which can make it easier but then u need to use a crossover to seperate the signal
If you are gonna go to the trouble of fitting an amp... then you need to replace all factory speakers and the head unit is a joke compared to a good aftermarket model from Pioneer, Sony, JVC, Kenwood, Clarion, Fusion, Eclipse and others I am sure that have at least 2 RCAs output, but 4 is much better (Front and rear).