Do wut i did buy a 25 dollar thing at best buy(not sure what its called) it is a box that on 1 side you connect it to the back of your stock stereo they are wires to hook up to the speaker wires and the other side is for your RCA's.
there's a thread a couple of posts down: LOC, or in other words Line Output Converter. It's exactly what your looking for and has some great insight on the LOC vs a new head unit. Go check it out ray.