what you are gonna have to do i know this cause i had to do it it sounds tacky but trust me get a couple rolls of the actual alumium duc tape $6 a roll max and tape the harnesses the rca cables and the whole inside of the dash where the radio is and the squeal will stop i got this fix from a mechanic who did stereos for a living working on corvettes and he said that they had this problom all the time when they did the aftermarket headunits.

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shut up, buckle your seat belt and hold on for dear life I wanna try somthing!