iPhone Navigation Over Onboard Speakers
It is possible to get iPhone GPS navigation prompts over onboard speakers when connected via bluetooth (mind you this is only possible if you have enhanced bluetooth).
In addition, if you are streaming music it will mute the music to broadcast the navigation prompt, very nice and quite polished execution.
I've also tried the same thing with my Garmin 2495LMT GPS, however, I was unsuccessful since it does not support A2DP via bluetooth. Any suggestions for a work around?