how about instead of all this grounding ******** - get a portable media device like an ipod, and get a 3.5mini jack to rca adapter. then first remove the headunit, and get some female to female RCA adapters and plug the rca from the media device into the RCA you have running through your car to your amp using the female to female adapters, this will test to see if there is any backfeed through your RCAS into your headunit. if you hear noise, either your amp or RCA wires are messed up. if you dont, then its got something to do with your headunit or backfeed through your rca to your headunit ground.
here is a quick image for you.
what this does, is 'decouple' your media output from your ground circuit. aka- the frame of the car. what usually happens with ground noise, is that the ground goes back through the RCA's into the headunit and goes out the headunits ground to the battery because the ground path from the amplifier is not adequate, not saying that the ground you made was bad, but the path from the ground TO the battery is is not large enough so it trys to find the least resistance through the body panels of the car, and often the tack welds that connect them have too much resistance.
if you still hear noise (wich is 99% unlikely) than it is your RCA or your amplifier. to test this, just hook the ipod right into the amplifier and leave the rca installed in your car out of the circuit. if you still hear noise its your amplifier.
if you do NOT hear noise, you either have ground feedback through the RCA to the headunit to the battery, OR you have a problem with your headunit. see if someone will let you use their headunit to test.
some people have said you can wrap the ground of the RCA at the back of the headunit with some wire, and ground that - and it has solved their problem. from my recollection this has only worked with pioneer heaunits and has 'since been solved' but who knows.
hope that helps.
