I am in a similar situation in that I have a Garmin 660 rather than a Garmin Road Tech 660.
What I have arranged with the Dealership is to fit a clutch control mounted Zumo bracket and wire that up to the bike battery using the Garmin supplied cable that comes with the Garmin supplied motorcycle mount. That cable has a load of other plugs / sockets on it (headphones, mike, traffic etc) and they will be hopefully sited under the seat. Unfortunately the cable can not be internally run inside the bars as its way to bulky.
I presently use the 660 Bluetooth capabilty streaming to my custom made ear plugs with built in speakers. These ear buds plug into a small bluetooth receiver I have in my jacket. the 660 streams info to the receiver via blue tooth and the speakers in my ears get the instructions.
What i will do with the system when i get delivery of the bike in November, is to plug the blue tooth receiver into the front panel Auxilliary input of the head unit using a small mini headphne male / male cable. The bluetooth receiver can go into a screen pouch pocket or somehere, whatever. So when the 660 streams information it will play back through the bluetooth receiver connected to the head unit as long as it is switched to Aux. If I want tunes I just go back to radio or ipod and use the GPS screen display to navigate by. I only use the GPS voice instructions once I get into the area I am not sure about. For general traveling I forget the voice part and just use the 660's dispaly screen.
Obviously the bluetooth receiver would have to be charged occasionally. The receiver I use is tiny but powerful, made by Jabra model number BT3030
Make sense?
Ride safe.