I have aftermarket multimedia stereo and installed a backup camera but the gearbox switch wire from head unit was not connected by stereo shop(bc I did not have the camera then) and need to know where the gearbox switch is and the wiring so i can connect the one wire from headunit to it to activate camera when I put car in Reverse-anyone know or have diagram? Camera is connected in back to reverse lights and works fine when i manually select camera from headunit.