I thought I'd share my findings and solution to this issue I've had after swapping one Pioneer head unit to another. My original HU was a Premier and the replacement is a much cheaper, standard LCD Pioneer from the same era. Though they're basically the same thing. Ironically I think the cheaper replacement is more attractive because it doesn't feature the ugly aqua blue OLED, but that's a different story.

Anyhow, since the replacement was basically identical, aside from the display and lack of MP3/WMA support, it was a direct swap down to the harness. Initially I didn't notice the issue, but with the replacement, virtually all noise in my electrical system was audible through the subwoofer. Voltage drops or changes due to turning on/off headlights or brake lights would produce a quiet thump or pop. I won't go into the other systems that were producing noise because they're specific to my car, and the source of the problem was the same regardless.

Obviously thought it was a grounding issue, I had the original deck grounded solely at the harness, and kept it that way when I installed the replacement. So I finally ran a separate ground to the deck chassis, fully expecting it to solve the problem, but any improvement was minimal. I then moved on to isolating the stereo system from some custom work, connecting it directly to the battery, grounding the amp and deck at the same point, swapping RCA's, even tried using a secondary inline noise filter on top of the one that's built into the head unit. None of it worked. I ended up testing the original deck in the car to see if the problem went away, which it did, so the lower end replacement Pionner was obviously the culprit, which I already expected since nothing made the symptoms go away aside from removing the RCA's to the sub amp or turning off the stereo itself.

I decided to give supplemental grounding another shot, at first thinking I might have to ground the sub preouts to the chassis, which didn't help at all. On that note, I tried touching the grounds of the unused set of front outputs with my chassis test lead and the noise went away entirely. So, I put this together, finally making use of a set of cheap Radio Shack RCA connectors I bought years ago and never used because of the quality. I usually sacrifice perfectly good RCA cables when I need the connectors, but it didn't seem worth it for this application.

http://i113.photobucket.com/albums/n...a.jpg~original

http://i113.photobucket.com/albums/n...b.jpg~original

So I suppose if I were a standard consumer, I would have been upsold at this point by a salesman telling me that I need a more expensive deck that has better noise filtering capabilities. Fact is, the Premier did not have the problem, in the same car with the same configuration, yet the cheaper Pioneer does, even though they are virtually the same exact deck. Regardless, this is all it took to solve the problem, ground the unused set to the chassis. Maybe this will be of some help to someone having the same issue.