I have been trying to do this for about 8 months now on my 03 Ultra. I have tried every combination of cable from the 2610 to the aux port that I could find and I even tried some home made ones. I have talked to MANY people about this and no one has a solution. I have found only 2 ways to do it and neither is satisfactory.
First you can disconnect one of your speakers from the factory radio and hook your Garmin directly to that speaker. That way you have 3 radio speakers and one GPS speaker. This works fine but I don't like losing the stereo effect and it would be really bad on a Road Glide.
Second you can get an earbud speaker from a cell phone and splice on a 2.5 mm mono audio fitting to plug into the Garmin. I can't hear it well enough to use it this way but it does work. If you turn up the volume on the gps it distorts the sound in the ear bud.
I would think if it was easy to do Kennedy Tech would have done it but they told me that they had no intention of making something for this application
Please post something here if you find a good solution.