Is this the kind of thing you are looking for?
http://goldwingdocs.com/forum/viewtopic.php?f=4&t=162&hilit=bluetoothAdded a bluetooth head set hard wired into his GW. I know that head sets now can transfer audio also, besides just phone calls, so you should be able to hook up your bluetooth mp3 player to your ride.
My new bluetooth has an on/off switch, so to hardwire into power should be a concern. I bought some hardwired water proof USB plugs, I plan on adding onto my bike, and my head set charges via micro bluetooth.