You might check the volume levels on your phone for play back. I had the same problem, but on my navigation via bluetooth the phones vmedia volume was turned down.The turn by turn instructions from google maps on your phone, via blutooth.
I understand it isn't going to show the google maps on the HU screen, but the audio should go through the speakers.
I have a Samsung Galaxy Note II dont know if what I typed earleir applies to iPHONE 4SI tried it and was only able to get turn by turn using the aux cable but not bluetooth. This was on an iphone 4S.