I'm using StreetWires Zero Noise 5 RCA cables (twisted-pair). I'm pretty sure it's not coming from the RCA cables because I completely pulled the RCA cables out and ran it straight back to the amp through the interior cabin and it still had the same problem. Also tried regular RCA cables (when I tested with the shorter harness plug where I relocated the head unit in the trunk for testing).

At this point, I'm pretty sure it's a grounding issue with the head unit. The problem is that the ground wire is tied into the Fakra cable. So that means that it's running from the front all the way to trunk (~18 feet). Wonder if this length is too susceptible to noise being picked up? I've tried cutting the ground cable at the Fakra harness in the trunk and re-routing it to the chassis and also to the amp's grounding post without much success.

I'm wonder if the ground wire run is just too long? Should I try to cut the ground cable at the head unit harness' end and grounding it somewhere up front? I can't seem to find a good place upfront to ground to. Any suggestions on a good grounding point up front? I don't know what those metal pipes are behind the dash where the head unit slides into.