On my LT I have the 2820 and Voice II and everything works over wires. On my RT, however, it's a different story. On the RT, I bought a 2720 (I got it pretty cheap so went for it) GPS. I have an aftermarket radio (public radio, CD, etc.) installed rather than the stock model. And I use a Windows Mobile 6 Pocket PC phone (phone, MP3/WMA, video, e-mail, internet, you name it this thing probbaly does it). To use the phone on the motorcycle I bought a Camos 600 BT headset.
Normally, I only used the Camos 600 so I could answer the phone while on the road. As a consultant, I have to be accessible 24 x 7. Then it dawned on me that not only could I use it for the phone, but I could tie in the GPS since my Garmin Motorcycle mounting kit included a power cable with mono-audio output. So I started searching for a BT output from a 3.5 mm input. Here's what I wound up ordering from eBay ...
I went with this one because it offered AD2P BT profile so I could use it on stereo devices. I didn't realize at the time just how important that was going to be, until later when I tried to pair it on the secondary (mono only) channel on the Camos 600. It would not work, but the primary channel on the Camos 600 supports two AD2P pairings. So, my phone is one, and the adapter is another. Both work. I can hear the GPS. Plus I can use the phone. It overrides the GPS. I did not try to see how it would work when playing music, but will later just because I'm curious about it.
But the bottom line is with the Camos 600, my phone, music, GPS, and intercom (if I add a passenger) are all accessible and functional.
PS: I did buy a 3.5 mm stereo to 2.5 mm mono adapter to plug in between the BT dongle and the output from the GPS. I didn't realize that the GPS was a mono 2.5 mm output.