Aftermarket head unit popping. Help please

[Fiddy]

So, I've got a '96 A4 B5, and I just put a pioneer deck into it because they stopped making cassette tapes a couple years back. Even though I've used harnesses and such for hooking up my deck to the existing bose system, I'm getting a wicked popping noise when I change stations or adjust settings, etc. From what I've read, I just described a ground loop. I've installed a dozen or more aftermarket decks for myself and friends, and this is the first time having this problem. Before anyone starts suggesting anything, I'd like to make clear what I've checked so far: -I've checked all the grounds. Everything seems to be properly grounded. I've even tried grounding the headunit to the chassis with a separate wire. No help there. I havent spliced into the ground wire for the factory amp yet though. -I've read that pioneer decks can have an internal grounding problem with the RCAs. So, I've regrounded the RCAs to the HU. No help there. -Wires all seem to be in good shape. I cant reroute the wires to the amp with shielded ones, but I figure that if that was really the problem I would have had the same popping issue with the factory deck. -I havent tried ground loop insolators yet. They have been called a "cheating fix", and doesnt actually solve your problem but rather just masks it. Are there any audio gurus out there that can help me? I'm getting tired of troubleshooting this. Thanks