Ok, just to be clear -- you're concerned about the Bluetooth connection with the car system, and not with a headphone connection, right? (In your original post, you said, "How do I turn the headphones back on?")
Does this persist after a reboot? Have you already tried unpairing the device from the car, then going through the pairing process again?
When you're in the car, are there any other Bluetooth devices that might be competing with the car system for a connection (like a Bluetooth headset)?