My vote...
Amp had a bad ground and 'found' a supplementary ground thru the RCA shield.
Seen this happen before. Actually sold an Alpine MRP500 and a Pioneer deh1300 to a customer last week. I installed the HU, the customers cousin went back in and installed the amp w/ an 8ga kit they also bought from us. I did the HU install tuesday, the customer was back in thursday saying the FM radio would not work. I didn't know the amp had been install, but noticed that the trim ring on the HU was gone and some moron tried to pry the HU out of the cage rather than use the radio keys provided in the HU's box. I pulled the HU, sure enough the Ant was unpluged and I found RCA's and rem wire taped (w/ masking tape) into my nice clean HU install.
I checked over the install, it was the usual door-jam power wire. No ring terminal to batt, just wire shoved under the ford batt post then the stock terminal pounded down over it. I get to the trunk to check the ground....um....yea...what ground? The amp was on and playing, but there was NO GROUND WIRE on the amp's terminal. It was not grounding thru the case (it was on carpet), it was getting its ground thru the RCA shield. I pulled the RCA's, amp went off. I then explained to the customer that due to this crap install, their nice new Pioneer DEH1300's warranty was void, and that I had know idea how the HU's outputs survived like that. The amp was capable of 50a+ current draw.
