Hello fellow frustrated stereo installers!
Well I had some pretty miserable alternator whine. It was constant and got higher in pitch as I accelerated. I tried re-grounding down to bare metal, new ground points, re-grounded the battery, tried different RCAs, etc. And nothing worked. Here is what fixed it for me...not saying it will work for everyone but it did the trick for me when nothing else would. My alternator whine was most prevalent in the front left speaker with some whine from the right.
I had purchased a PAC ground loop isolator that I tried putting on the amp and all it did was make the system sound weak...making the "noise" quieter but just because it was quieting the whole system down.
After grounding each RCA and re-grounding the deck and still not fixing the problem, I tried putting the GLI (ground loop isolator) onto the 2 front speaker RCAs, this got rid of the whine, without making my system weaker in the process. It sounded exactly the same, minus the whine/noise.
However, when I put the deck back into the hole in the dash the "noise" came back. Here is the quick summary...by taking all of the excess RCA cables and isolating them from all the other speaker wire (by zip tying each group and then wrapping them in closed cell foam and putting them as far away from each other as possible) it eliminated the alternator whine completely.
I assume that the GLI helped with some ground loop in the RCAs and separating the cables helped remove the interference they were causing each other. So if all else fails and you are getting close to going into a black-out rage from your alternator whine, give it a shot. Doing this dropped my blood pressure by about 40 points every time I am in my mREX.