my 98 mountaineer had the mach sound system with the rear subwoofer but the display on the headunit started going out so I decided to toss one of my alpine headunits in the truck. The aftermarket harness only used one harness as the other two were reserved for the sub and console cd changer respectively. Well, I had the sub and wanted to still hear it. I looked up the wiring diagram on mitchell-on-demand (which I have) and got the color codes and went to work. It's pretty straight forward. I just got some quick connects for the wiring purposes and some electrical tape for the connections from the RCA cable. Oh yeah, just get a spare rca cable and cut it open to get access to the positive/negative wires to splice in just in case your headunit has RCA pre-outs for a sub. If not, you can get a line-level-to-rca converter and then get a passive sub crossover so the sub amp wont get a full range signal. Anyway, here's the connection 1. connect the Yellow/black stripe wire from the main harness (switched power) to the teal/purple stripe wire from the 4wire amp harness (power wire). 2. connect the black/green stripe wire from the main harness (ground) to the black wire from the 4wire amp harness (ground). 3. connect the two wires from inside the rca cable to the red/black wire (sub speaker positive) and the brown/orange wire (sub speaker negative) from the 4wire amp harness. Then just hook the rca connector side into your sub pre-out, toss the stereo back in the dash, and set up the sub level on your headunit and you're done! I can honestly tell you this, the little 6" sub from my factory system hits a good bit harder now and the signal is cleaner. Probably due to the better pre-amp signal it's getting from the alpine headunit and it's got all the added bass I need for this truck. I figure this may help others out who have been wanting to do this since...well...you've already GOT a sub, might as well use it!!