A friend of mine recently installed a Terk XM tuner in conjunction with his Pioneer Deh-p8600mp reciever, RF amplifier, and RF sub. Now, whenever he turns the car off or changes the radio stations, there is a thud which seems to be coming form either the rear 6x9's or the subwoofer (my guess is the latter).

I have checked the ground, and it is short, so I don't believe this is the cause. I changed the RCA's to XLN PRO's, and still noticed feedback, so it is not the RCA's. I also checked all other wiring to make sure that there was no short circuit or break in the wiring.

Further, we took out one component step by step to see what the problem was, and when we took out the RCA's from the deck to the amplifier, the feedback stopped.

What could be the problem?

CLiff notes: Deck seems f'd up, someone please explain why.