Music always playing when in car

HI I have a 3gs, which i have connected to the bluetooth in my car, A ford Kuga.
The problem is, as soon as i start the car up, the bluetooth connects to my iphone and the music automatically starts playing. If i am listening to the radio i have no idea that the music is playing, until i look at the phone and see that it is playing.
i then have to manually turn the music of on my phone.
This drains the battery somewhat as the phone is basically permanently in use.
Surely it makes sense that the music only plays when i select the iphone on my in car music system.
Any one have any ideas how i can stop the music playing automatically...

Thanks
Rob