I've got the original JoyCon (not ex) and I've tapped into the factory wiring for the steering wheel controls. The unit works well when it is connected to the carpc but if you have to take the carpc out for whatever reason I lose all control of the headunit (ie I can't change tracks, radio stations etc).
I've also noticed that if I leave my radio on and turn the car off, when I go to start the car again the volume goes up automatically to maximum volume. There's a bit of a delay as my carpc boots up and I think it's during this time the a resistance is being transmitted that matches the volume up signal. I just turn my radio off whenever I turn off the car now.
I hope that helps. The installation is in a 2004 Mazda6.