Can I have my iPhone navigate through my car's stereo?
Can your car accept the iPhone's music audio as an input to the stereo? On my care (Acura TL), the bluetooth profile lets me play iPod music through the stereo when the phone is paired. As a result of that, ALL of the phone's audio plays back through the stereo... including text alerts and any apps that play back voice sounds (navigation, etc).

It's probably too simple an answer, but it's worth checking to see if you can pair up using the audio profile for music -- in which case you'd get the phone's other audio as well.