I helped install a Directed 4 channel amplifier in a Honda Civic recently. There was already a larger mono amp in there for driving a subwoofer so we tee-off the 4-guage power wire running to that and brought it over to the new, smaller amp, as well as the ground and amp turn on wires. RCA's are running from the back of and Eclipse head deck into the new 4 channel amp and it works excellent. However at idle there is a noticeable high-frequency engine whine coming through the speakers now. We noticed that by removing only one set of RCA's (either one) the noise would stop completely so we figured there to be a grounding difference between the two RCAs so we put a ground loop isolator on one RCA.
It seemed to help a little at first, but now it's still apparent and I would like to find the real culprit in this situation. I can always toss more GLI's on but that seems like a backwards way to fix this problem. Both amps seem very well grounded and the RCA's are new and of high quality. What's next? Could there be an issue with the head deck grounding?