if your connecting it bluetooth, the audio for the navigation won't come through on that, it's only using the bluetooth headset profile & will only work for calls & voice recognition maybe for contacts voice dialing.
you should be able to interface the nav audio through the headphone jack, would also require providing an auxiliary input. this isn't a huge project, but then the drawbacks would be that in order to listen to the nav you would have to tune to your aux source & won't be able to listen to music or anything else while your listening to nav. it won't work like an interrupt when instructions come only.
I'm sure it could be done to work nice with voice interrupt too, but the amount of work that would take, it would start to make more sense to just look for another better nav solution I think...
best easy solution might be to just get an aux speaker to plug into the phone, like a set of portable ipod speakers or something, maybe clip to the visor... then the audio for the nav will at least be usable, but independent really of the vehicle system itself...
