the ignition harness and theplace it plugs into. really easy...i installed mine myself in about 15 minutes.....i didnt hook up the boost gauge function though...but yeah, the harness is definantly worth the extra 20 bucks. just unplug the old one, plug this one into both ends, and ground like 2 or 3 wires, hide the big module and mount the little one somewhere...very simple..the instructions are allright too that come with it. i beleive it will fit in the clock space. what someone has done before is to chop off the brackets on the sides of the clock and superglue them to the sides and just screw it in like the clock and voila! and it has absolutley nothing to do with your stereo