I take it you aren't removing/changing your HU, so that makes it pretty difficult to hook anything up to it. If you went out and bought a cheap(ish) HU that has at least RCA inputs you could just hook up to that. Otherwise your only option would be some sort of RF transmitter like they make for iPods, where you tune your radio to 89.9 or whatever. The sound quality/reliability on those sucks though.
What kind of inputs does the stock amp take?
What is the rating on the stock speakers? (ohm, watts)