The sound you are hearing is a ground loop, and the isolator will remove or quiet it down. You need one that just has a wire on each end so you can put it in line with your ground. They make them with fittings on them for different devices such as rcas to go inline with your amps. Be careful what you buy though, some of them work well, and others don't work worth a damn.
I was helping with an install in... some crazy car. Forget what it was now. Anyways, no matter how we grounded it the whine was still there and it was bad. We had two different brand of GFI, one we used on the left rcas and the other on the right. Left side was quiet, right side still pretty loud. Switch them and the noise switched. Hence my warning, some just don't do a whole lot while others are pretty good.
COTY - 2012
Tom - 03' Bonneville SSEi', 00' Sierra SLE, 1980 Goldwing GL1100 Interstate