Manufacturers have installed automatic headlights on cars for several decades. Originally offered on expensive models, they have since become commonplace. New cars are equipped with electronic modules to control the entire vehicle lighting system that monitor and regulate systems such as the interior lights. They stay on for a few seconds after the door is closed rather than go off immediately. There are two options available when adding automatic headlights: install a factory package or an aftermarket kit. If the car manufacturer supplied the accessory kit to retrofit the vehicle, the switch will be an original equipment unit with added functionality. The aftermarket kit will contain a toggle switch or may integrate this function into the sensitivity adjustment.
Mount the photosensor in a location where it is exposed to light. Factory units are installed on the dash pad. Use a 1/4-inch socket set and screwdrivers to remove the lower dash to gain access for installation. You may need to remove the upper dash cover as well. Use double-sided tape to secure the photosensor in place.
Mount the on and off switch. Install the control module/switch in an inconspicuous location, but accessible enough to operate the switch. Inside a console, under the edge of the lower dashboard or in a small dash compartment are all good locations. Use double-sided tape if possible, or mounting screws if the kit manufacturer supplies them.
Wire the module according to instructions. This will include attaching the photosensor input wires as well as the voltage and ground wires. Route the wires from the photosensor to the module in a secure manner, and use wire crimps to install wire connectors to join the wires. Use a test light to locate a 12-volt power source that is available when the key is on--and off when the key is removed. Do not select a small wire; instead, choose a heavy gauge wire coming from the steering column. Be certain the manufacturer supplies fuse protection for this circuit. If not, install an in-line fuse that is rated for the module.
Crimp the wires in place, making sure they are tight and secure. Connect the module ground wire directly to bare metal, or crimp the wire into an existing ground circuit. The factory module has detailed instructions and comes equipped with connectors that are designed to plug directly into the existing wiring harness.
Locate the headlight and running light wires. Raise the hood of the car and find the wiring for the headlights and running lights that connect to the module control unit. This step, required for the aftermarket kit, is vastly different than the factory installation. The factory installation is completed inside of the vehicle and nothing is done under the hood. After the factory sensor, module and switch are installed, the job is finished. The aftermarket installation requires a direct connection to the lights. Turn on the low beams--not the high--and use a test light to locate the power wires.
Install the wires, leaving the module to power the headlights and running lights. Attach this wire as close to the headlights as possible and use wire crimps and connectors to secure the connections. Route the wires safely, keeping them away from hot exhaust systems and moving parts. Join the feed wires to the lights with crimp connectors and make sure they are tight and secure.
The headlights need to be adjusted at dusk. Set them to come on as the sunlight fades. They will turn off under bright lights, such as at a service station, if the sensitivity is set to do so. Light Emitting Diodes (LEDs)--used in automatic headlight circuits--are commonplace today. You can purchase a low-cost LED circuit equipped with a variable resistor to adjust the light sensitivity as part of a headlight kit.
Do not wire the aftermarket light kit into the headlight wiring as it just begins to leave the headlight switch. Do wire the kit into the wiring close to the headlights. Located between the switch and the headlights is an electronic control module that could be damaged by improper wiring.