I have a nuvi 765t and works great. It's always in an aquabox so not to be exposed to the elements.
Marcster actually showed me the FM transmitter trick during the Burger Run and it works great. Yeah, the music quality isn't 100% but perfectly fine for me. I have the music being piped through my cell and wear Skull Candy ear bugs while I ride. The music stops for the gps turn by turn directions.
What size box do you have, and do you have it hardwired? I have the RAM-HOL-AQ6U (Medium Wide, says it is for Garmin Nuvi Devices) and I can't plug the power in. I think I need a right angle adapter, or a right angle cable.