Can anyone help me out? I installed a system in a friend's 88 F150. He bought everything brand new. We are running dual amps, with a Pioneer head unit, new door speakers and he added two 6x9's to the top of his box in the back. (His F150 didn't have any rear speakers factory) I have regrounded everything several times, used several different RCA cables, and have ensured that the power and signal wires are not routed anywhere near each other. Is there anything else I can do to try and fix this problem? Could it possibly be a bad ground somewhere else in the truck not relating to the speaker install? let me know if you can please. Thanks.