For identifying whether the car door is closed or not. Door light switch will automatically open the alarm and flashing function. The door will be locked when you close your door if you do not install the door light switch. Function: for anti-theft device identify whether the door is closed, if installed door light switch can achieve alarm device automatic arming function and open door flashing function. If you do not install the door light switch, then the car will automatically shut off after the latch, lock the door, so it will be very inconvenient, if the door is equipped with a light switch, it would avoid such a situation. This is a non-OEM product. Accessory only!