Sorry I just noticed this thread. Did you get it sorted out? One possibility not yet mentioned is RFI from the ignition system. It gets in through 3 points: the power supply, the RCAs, and the antenna. On mine the majority of the noise was in the power. I installed a noise filter designed just for that purpose, easily found even at Canadian Tire, There was still a little bit of noise, so I assumed it was in the RCAs. Installing an amp creates a ground loop in the RCA lines. For this you use a ground loop isolator, also easily found in stores. You just put it in line in the RCA wires to the amp. In my case it was a DVD player so it was input not output but that makes no difference. You have to be careful to place the isolator out of the way of any other RFI source or it will pick that up and put more noise in the sound, so keep it away from the instruments, any HU, amp, speaker, iPod, who knows what, just keep it far from anything that is electric. Well that all solved my noise so I did not proceed to the antenna, but thay also sell a filter for that.
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