I have an '01 and I can't imagine that the radio wring to the speakers is much different. I have had issues with garbled sound and that is a poor connection in the rear of the radio. Looking at that problem I saw many connections between the radio and the speakers. Since it is intermittent you either have a poor connection in a plug or a chaffed . That usually eliminates the radio, but not always.
You could do two things. Turn the radio on and follow the wires back to the radio lifting them looking for a failure or sound to return. If both rear speakers were out I would suspect a single ground issue but if it was only one you are pretty much hit and miss until you can make it work or fail. I would not tug too hard to break a or make it fail permanently. If it doesn't work at all it is much more difficult to fix.
Thanks for the heads up on the speaker wire. My first "to do" will be to clean the connections under the back seat and then follow the wires back to the radio. I think you might be right about the chafing of the wire...I did the 12k maintenance of the bike this winter and who knows, I might have moved a wire in an area that caused it to become exposed to a metal part. But then again, the back speaker is either "working" or "not working."
I have a 2002 LTC and the intermittent speaker issue started with my left rear and then I lost both front and now have only the rear right speaker... from my research the problem is the radio. The amp goes out it this fahion and requires a new radio.
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