I use this bluetooth adapter in the car:
Toothpik Hands Free Bluetooth | eBay
I've had it for a couple years and several phones. On my S3 the bluetooth will not automatically connect, I have to bring up settings and go to bluetooth and then settings for this devide, tap on it and then it will connect. Every other phone, including my previous Galaxy Nexus, always connected when I turned bluetooth on. What's the deal?