I have the exact same problem on my 2001 GL1800. I've tried everything but seems that radio is gone and needs to be replaced. I've ordered new unit on ebay and waiting for it to arrive. Have yu been able to resolve your problem?
The radio on my 1500 SE, stopped working after I connected the amp. that was installed to the on/off power wire on the wiring harness. The speakers will not play, however I am getting via the intercom.
I was pushing the mute, it came on for a moment, then died. I turned it on, on another occasion it played a moment, then quite.