is it an aftermakert head unit?
maybe i should put it this way... no matter what, u need to buy an amp wiring kit, this usually consists of a low gauge (thick) power wire with a fuse that should be placed close to the battery of the car. It should also come with a higher gauge (thinner) wire that is used as a remote (to turn on the amp when it recieves a 12V + signal). it should also come with a set of RCA cables to hook the amp up to the stereo.
to install the amp, you should drill thru the firewall and wire the thick wire with the fuse close to the battery. but dont connect to battery until you are completely done with everything. this power wire should be wired to the 12V-input on your amplifier. this is the step you need to do with every amp. also dont forget to wire the amp to a good ground source in the trunk or chassis.
For an aftermarket head unit, there should be RCA AMP outputs on the back. all you need to do is conect the rca output from the back of the head unit to the rca inputs of ur amp. and in the wiring harness that attaches the honda harness to the stereo harness, there should be a cable that says "remote out". you need to connect this cable to the thinner rremote cable from the amp wiring kit to ur amplifiers "remote". what this does is when ur stereo turns on, it sends a 12V signal to ur amplifier simply put.
i hope this helps out