I can't directly answer the question as to how to use Bluetooth to transmit music, but I can tell you what I do at a small cost.
I get a FM Transmitter for the car. Plugs in Cigarette Lighter for power, attaches to headphone socket of the Blackberry, and then it transmits the output of the Media Player over FM to my car stereo.
Best bit I like is I can have a old looking stereo in the car that no one wants to nick. AND this comes with me to any hire car I use or friends car I jump into. Ideal.