You can use that setup and it will sound really loud too. What you have to get is a converter that can convert AC to DC. The DC current is then fed to your amp which will power your subs. RCA's from the stereo will plug into the amp as well. It's a pretty simple set up, I have seen it done, and I have heard how loud it can be.

EDIT: Nice, posted at the same time...

___________________
Helping girls make bad decisions since 1979

Sorry guys but if you plan on using a 500 watt car amp, your going to need one serious 12v power supply with enough amps to power that thing, and there not cheap. You can use the speakers, but get a good pro dj amp...it will cost the same to buy a power supply you need, and use it to power it instead, and your other speakers with power to spare. Keep the car amp in the car where it belongs.