w126 ground loop noise through speakers

[BenzTurbo]

i am installing a new stereo in a 1991 350sdl. it originally had the "premium" sound with 2 amps behind the rear seat. the new equipment is: mb quart 5ch amp, kenwood head unit, polk db speakers for rear deck and doors, mb quart speakers for dash, polk 8in subwoofer, and rockford fosgate 4awg wiring and rca cables. i extended the speaker wiring from each original amp harness to the trunk where the amp is. everything sounded great until i shorted one or both of the speaker terminals to the aluminum dash board while installing the driver's side front speaker. it mad a pop noise and now ALL electrical noise comes over the speakers. the clock, the seatbelt light flashing on the overhead dome light, very loud when you turn on a larger load like the hazards or turning the key to "run", and of course alternator whine when running. all speakers sound perfect though when you play music. in my troubleshooting attempts, i have tried:
-using 2 other known good amps
-using 2 other known good head units
-wiring head unit directly to battery using no original wiring
-cleaned amp ground at rear seat belt bolt again
-cleaned chasis grounds in battery area
-cleaned all positive wires near battery
-checked and cleaned all fuses

the only thing that seemed to help was making a cable go from the stereo's body to the outer part of the rca plugs. if i unplug the rca's from the stereo, all electrical noise goes away and i can plug them right into my phone and listen without any issues or noises. any ideas what could cause this??? everything was fine until that speaker grounded out. i'm guessing something on the car is causing all this because i have tried using all new equipment. i really need to figure this out because the stereo is pretty much unusable now.