The Google Maps app seems to work with giving turn by turn directions via Bluetooth. Every time a turn is announced Google Maps tricks my 2008 Altima Coupe into thinking its making a hands free call and announces the directions over Bluetooth. Siri seems to work the same way (after I push/hold the home button). However, if I have my phone plugged into the aux input on my car Siri does not work over Bluetooth.
It would be nice if Apple added audio source options to Apple Maps like iOS does with phone calls.