On my fairly new Fascinate the app Music Player turns on when I enter my car with the vehicle synced to the phones Bluetooth enabling phone calls. I don't realize that the Music Player is on until I leave the vehicle and hear the music now coming from the phones speaker. Of course Task Manager seems like the only way to turn off the app. But same scenario repeats when getting into vehicle.
Go back to the bluetooth pairing setting on the phone which you created for the car. You should have an option to enable it for phone calls, media or both. Uncheck the media option.
You can use A2DP Volume to manage the settings when connecting and disconnecting from your cars Bluetooth. It can also launch and kill apps based on connection. I captures your cars location automatically as well as several other features. Just search A2DP Volume on Android Market. It is free, open source, and ad free.