I hope this may be helpful to some - I had a horrible ground issue in my car. Setup was as follows:

12+ 4 gauge wire down the passenger side from the battery back to a distro box. Box split to 2 8 gauge wires to each amp. Signal into the amps came out of the head, down the driver side via 3 rcas (front, rear, sub), into the amps. Went out of the amps, via 9 wire cable in 1 surround back up next to the RCA's into the stock harness.

Ground was to the rear that shared w/the defrosters.

Had a pop/snap sound every time i hit the brakes, or turned on my turn signals. Tried movin the ground to the rear seat post, didn't work. Pulled the driver seat, ripped up the carpet, and ran the rca's down the center of the car ... worked. No ground pop or hum whatsovever. Apparently isolating my rca's and return cable back to the input stock harness fixed it up.

 

you should always run your rcas on the opposite side of the car away from your power wire.. because of the fact, which you experienced, road noises coming through the speakers

 

I did put the RCA's separate from the power. Power was down the passenger side, rca's and speaker return wire to the stock harness (so i didn't have to route every speaker individually off my 4 channel) went down the driver side. The "speaker return" wire was a 9 conductor wire if you will. Each wire separately insulated, then the entire bundle insulated (4 speaker + - and a remote turnon lead). What I was saying sometimes just isolating the power away from the rca/speaker wire isn't enough.