well he did say that he had a Rockford amp, (OMG - Im gonna piss someone off) - anyways, Rockford are notorious for picking up all the noise that exists in your car. First things first, as previously mentioned - Check the ground.
I dont know how many times Ive had a car come back to the shop I worked at and the whole noise problem was a bad ground
Second, If it is an Xplode series deck, than I bet the deck is on its way out. Funny thing about Xplode's - They live up to thier name
Here is an installers underground trick. Hopefully you have an old walkman with a tape deck in it, dont put a tape in it and press Play. With the car running, put the headphones on, and move the walkman around where you put your RCA cables and you will be able to "trace" the source of the noise. If you get a lot of noise in one area, move the cable away from that area.
Also, try replacing the glass fuse under the hood, I had a fuse that was improperly assembled, and it cause all kinds of noise, i replaced it and the noise went away.
If your still having problems, email me -
drivett@eagletechnology.net and i can help you..