Don't use the built-in map/directions on the iphone, it is notoriously unreliable and flakey.
Try Google Maps:
https://itunes.apple.com/us/app/google-maps/id585027354?mt=8
Or, even better, Waze
https://itunes.apple.com/us/app/waze-gps-maps-social-traffic/id323229106?mt=8
Also, make sure you're in "Bluetooth Streaming Audio" mode
O_E