I have a jailbroken iPhone and can run an app called bluetoothmono that sends sound from you iPhone across the same way as a phone calls audio.
but... IT SUCKS.. converts you nice stereo music to mono, and the bluetooth hands free phone in the car only outputs to the driver side speaker, not the entire car...
HERE IS A SOLUTION
..haven't gotten it yet... but it's on the list...
It is a bluetooth audio reciever that you can plug into your aux jack, there might be cheaper ones out there...
http://www.amazon.com/Belkin-F8Z492-Blu ... 038&sr=1-1