I don't think this is strictly a problem with the 920 but more problem with the bluetooth profiles equipped with the car stereo.

My wife's 920 pairs up with the car (a VE Holden Commodore) OK and works fine for making and receiving calls.  The problems arise when she wants to use Nokia (Here) Drive.  The turn by turn directions don't get played over the car speakers nor the phone speaker.  

When the 920 is used in our other car which has a Parrot bluetooth handsfree kit the turn by turn directions play through the car speakers just as expected.  This makes me think it is an issue with the stereo in the Commodore.

Does anyone have any idea on how this problem could be solved?

I haven't tried this but reading on some other sites where this problem occurs, streaming music to the car stereo from the 920 allows the turn by turn navigation direction to be heard. I don't think this is  particularly practical solution.