RE: Adding new amp/subs to factory radio in my '03 CTS?
Hey there. Yeah I did it to my 06 CTS, and kept the factory radio and everything. I have 2 12" Treo subs and a Zapco amp. The rear stock sub in your car should be a dual coil sub, which is a plus. All you have to do is disconnect the battery connect and run your red power wire through the fire wall. there is a couple spots, so you won't have to drill, I ran mine through my kick panel, through the rocker panel foot covers, then under neath the rear set, out through the trunk. Hook that to your amp. run your speaker wires from your amp to your subs. Then for the remote line, insead of having to run it up to your radio you can pull your back seat off and there is a fuse block under the rear driver side section. run your wire to the Audio fuse which will show you where its located in the owners manal. Also make sure you buy a peripheral controller from any audio store for like $15-$20 piggy back off of your stock dual coil sub to your peripheral controller and then to your subs. that's pretty much it, this way you get the same result and don't have to pull the radio. One suggestion for you though. I would get an aftermarket radio/Cd player, because it does not sound as good unless you have an aftermarket, trust me, my subs bump and my system sounds very good, but if your looking for dB and more volume you have to go aftermarket because of the stock volume limiter. Otherwise if you stick with stock it will sound good, but it could always be better