hooking up amp to factory stereo??

If you got the right kind of amp, you don't need to buy anything. Some amps come with speaker level inputs. If not, you will need to make a trip to an audio store and pick up a gadget. I think they're called "line-out converters" but i'm not certain. What these things do is convert your amplified signal from your headunit and turns it into a preamp signal. If this were me, i would go out and buy an aftermarket HeadUnit w/preamp outs. Not only will this make the amp easier to install, but it will make the rest of the stereo sound mo better.