I reason i want to leave fm-carkit on all the time is because it doesn't take resource at all. It should be default to ON since you don't want to keep checking it every time in case you receive a call while in the car w/ radio transmitter ON.

Edit: Ok let me correct...i don't think it's a good idea to turn it ON by default. I didn't realize that when it's ON, you can't hear the voice from the caller or when you make a phone call, you can't hear the voice unless you turn off the fm-carkit. I do think it's a good idea to turn ON the carkit automatically when radio transmitter is turn on. And OFF when radio transmitter is off. It's just a convenient way of not having to press both.