Start the Bluetooth pairing process on your car's stereo. This process varies from vehicle to vehicle and is sometimes lumped in with the hands free calling system. The system may ask you a few questions, and then prompt you to complete the setup process on your handset.
The entry in the Bluetooth devices list should read "Connected to phone and media audio." If not, then press and hold on the stereo's name until the contextual menu appears, then select Options and manually enable audio streaming by checking the box next to Media.