You will have to get a LOC (Line Output Converter) to hook up an amp while keeping the stock head unit. The BEST one out there right now is the JL Audio Cleensweep, but it's priced at around $300. I've seen and heard it hooked up in a buddy's car and it's great! But too pricey for me. I am using a Navone 4 channel LOC - the NE-774V, found here:
Navone LOC
The LOC grabs the wires going to the speakers from the headunit, and pushes the signal through RCA jacks on the other end. It's not the cleanest or best sound, but if you want to keep the stock head unit, then this is the only way.