Bluetooth Auto-Connect

My iPhone 3Gs was able to automatically connect with my car audio system's bluetooth unit. I'd get in the car and a moment later, a bluetooth connection was established. However, my iPhone 4S doesn't automatically connect. I have to manually go into Settings>General>Bluetooth and click on the device name and then type in a four-digit passcode. Not advisable while trying to drive. Anyone know a way to make the iPhone 4S play nice with my bluetooth unit without parental supervision from me?