The original setup was a Kenwood headunit (KDC-BT955HD) connected to a run of the mill 4-channel MTX amplifier. The amplifier drove the 4 front speakers and the 2 rear speakers. No extra subwoofers or anything.
Periodically the rear left channel would drop out. I found that if I went in the trunk and tapped or adjusted the gain or adjustment knobs it would come back in for a little while.
So rather than mess with taking the amp out and cleaning the pots I decided to just buy a slightly more powerful amplifier, a PPI (Precision Power) 4-channel amp.
During the installation I found that the wire going to the rear left speaker was damaged and partially exposed/disconnecting.
After installing the PPI amp, connecting up the RCA's and then turning it the car on I found that I had alternator whine and all kinds of noise in the speakers.
Initially I thought it was the amp, so I swapped back. No change.
After some experimenting in the trunk, I found like 30V when I measured from the RCA shield to ground. That struck me as extreme.
I was reading on a few websites about weak ground connections on the RCA shield at the head unit. Their solution was to directly ground the shield thus bypassing the head units internal connections. I tried this and found that the RCA wire got HOT. Red flag right there. Was not sure what was causing it but I know thats not good.
I ordered a new Kenwood unit (newer model, upper end). I decided to pop off the back cover on the old unit to see if anything looked a miss.
I found a blown fuse that went between the RCA shield and ground.
What I don't want to do is connect the new Kenwood unit and just repeat all this...
Photos:
Kenwood KDC-BT955HD - RCA Shield Ground Repairs - Imgur
So yeah, nothing complex, nothing fancy or supremely powerful.