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.