Alternatively you might have some level of success by powering both devices from the same source though their relative positions on the bike might make this difficult.
Ground loop interference, so I'm told, is down the differences in impedance (and probably other factors) between the sources and earths for the devices concerned. I have an Autocom Super Pro Avi and a PMR radio powered from a battery saver, after advice from someone in the know, I powered these off the rear brake light feed and earthed them to the same point on the frame. I can't hear any significant GLI and neither could a mate on another PMR. Both Autocom and PMR live under my seat so that makes this easier. I do however have an Autocom isolating lead for my StreetPilot which I fitted before I knew much about GLI and it's causes, it was only £20 though.
