it doesnt know you put the car in reverse per se, it simply gets a +12 volt signal from the backup lights (which only come on when you put the car in reverse). Thats my own way of hooking it up. The other way is to spice into the electronic transmission wiring.... but i think we can just go with the first suggestion.... If you wire the power to the camera the same way, the only time the camera will come on is in reverse also. It really doesnt matter because thats the only time the dvd990 will display it anyways.