Okay, let's see if anyone can provide some insight on this. Here's my config:
2003 Monte Carlo SS
Head unit: Factory; AM/FM/CD
OnStar: No
Speakers: All 4ohm

I currently have the factory amplifier bypassed; the speakers are hooked up directly to the head unit. My car previously had a mono subwoofer amplifier in it without any issues (Alpine amp with high-level inputs). I got a 5-channel amplifier (Kenwood KAC-7005PS) cheap from a friend, and hooked it up. using the wires at the factory amplifier harness. However, there is some noise. First, I get alternator whine. Second, I get clicking from the turn signal and brake pedal. Third, I get some pretty generic constant static (minimal).

Now for the troubleshooting. I had grounding issues with the mono amp before, and I cleared that up. I double-checked again with the new amp and the ground is fine on that. If I have the amp turned on with no RCA input connected, the red indicator illuminates and I don't get any of the interference. As soon as I plug in the RCA inputs, the LED shuts off, the cooling fan starts running, and the noise begins. If I put a Radioshack ground-loop isolator between the RCA inputs, the noise lessens (but so does the volume), the LED stays on, and the fan doesn't run. So that leads me to a few questions.

1. Could the head unit be grounding the negative for each channel? This could explain the LED issue (electric short).
2. Could there be a bad ground under my dash? (I wouldn't be surprised, just not sure where to look)
3. My friend was looking at it for me and said when he unplugged my head unit with the amp turned on, the noise got louder. I'll have to investigate, but I'm not sure where that leads me.

 

What are you hooking the 5 channel amp to? A sub or your interior speakers?

 

I have four aftermarket 3-way speakers (front/rear), with my stock tweeters removed. The fifth channel is for a subwoofer, but none is connected currently.

I bet it'd be safe to say an aftermarket head unit with new power wiring would fix it, but I don't like the cheap look and feel of aftermarket radios - I've already invested in an auxiliary input for my stock radio.

 

Which PAC box are you using, and what do you mean your using the wires from your factory amp harness (your using them as the remote and speaker wire?)

Pac box should be wired in behind the radio to your FR FL RR RL, then RCA's to amp
if you have cheap RCA's that could be giving the feed back be sure to not run them on the same side of the car as the power wire
run a remote wire to a acc fuse (or any fuse with a toggle switch)
then run all new speaker wire back to the amp 12awg-10awg 100fr spool for about $30-$40

and go back to bypassing the factory amp harness