when you hook up a backup cam it will have a setting that asks you how to detect the reverse signal when its set to battery, when you put the car in reverse and the light comes on it triggers the signal wire to the head unit and turns the camera on and switches the screen to show the cam.. Also if you did the relay trick you can wire the camera power to a hot wire that is always on with the car and when your going down the road you can turn the camera on from the head unit and watch the road go by lol