Recently a company called BikeMP3 developed an MP3 player that interfaces with the GL1800 CD changer interface. It has gotten great reviews on the Goldwing forums so I decided to get one. They are available directly from BikeMP3.com for $130...as opposed to $700-1100 for a the Honda CD changer.
Here is the kit from BikeMP3...unit, interface cable, instructions.
Below are some pictures of the unit.
The player uses SD cards or USB flashdrives to hold the MP3s.
The CD connector is located under the seat and to the rear of the relays.
I ran the cable through the hole made for the CB installation. It just fit.
The cable is connected to the CD connector (pink arrow).
The cable is long enough to reach about anywhere.
I tucked the player behind my trunk organizer.
The player works great! You can use the CD controls to select tracks and disks (mp3 folders) with up to 99 songs per folder. The sound quality through the CD circuit seems to be a little better than the auxiliary circuit.
Update 2014 - The BikeMP3 player became less and less reliable so I replaced it with a GROM audio integration unit.