Just by chance did you try to ground your radio to where your amp ground is?
also if you have 2 battery's is each battery ground to a different location?
if so have you tried running a ground wire from main battery to Back battery?
I was told by a friend a long time ago that if you have ground noise that you should move all your grounds to 1 point and that will eliminate ground loop noise.
just a few thoughts anyway..hope you get it figured out, I know its a pain in the ass.