I think it's better if you turn on the fm transmitter, the carkit should turn itself on too rather than the other way around. And if i turn fm transmitter off, the carkit should turn off too. It's just natural to use the fm transmitter while in the car listen to music not having to deal with pressing another button just to listen to phone radio call. I think you got the whole design wrong dude.

Edit: i don't know what the hell i am saying or want lol

Ok...i think i know what i want....i want a widget or a shortcut on the desktop like fm-transmitter so it's easy....just one press.