How to Wire Fog Lights on a GMC Sierra


Fog lights are yellow to prevent glare. They should not to be confused with driving lights, which are extremely bright and reduce the vision when on in a condition with fog. Fog lights should be mounted as low as possible. Many year GMC Sierras have pre-existing holes drilled for driving or fog lights as an accessory. If not, a location should be found with ease of mounting and in a desirable low location in mind. When purchasing fog lights, get a product that comes with an inline fuse, a relay and a switch, all of which are needed during installation. If for some reason yours do not come with the kit, purchase the other parts at the same time.

Things You'll Need

  • Drill and assorted drill bits Phillips screwdriver Wire crimping tool Set of wrenches Voltmeter 4 standard yellow female blade connectors 2 yellow oval butt connectors 3 yellow butt connectors 2 blue oval connectors 2 blue female blades 2 blue butt connectors
  • Install the fog lights in the desired location with the provisions in the kit. Install a piece of 10-gauge wire with a yellow butt connector to the black ground wire at the fog light and attach it to a good ground on the frame with the yellow oval connectors.

  • Disconnect the negative terminal on the battery. Attach the relay in an easy access location as close as possible to the fuse and relay center on the driver's side fenderwell. Attach a piece of 10-gauge wire from the battery main input wire at the fuse and relay center to the 30-amp in line fuse and from the fuse to the fog light relay B+ terminal using a yellow oval at the fuse box and a yellow female blade at the relay.

  • Attach another piece of 10-gauge wire from the relay to the fog lights using a yellow female blade at the relay and t-ing both fog lights into a yellow butt connector. Attach a 16-gauge wire with a blue female blade to the ground side of the relay and to a good ground using the blue oval connector.

  • Attach a 16-gauge wire from the last terminal on the relay through the firewall to the switch using the blue female blades. Locate a straight blade terminal on the driver's side fuse block that has power only when the lights are on. Use the voltmeter to check for power. The harness from the headlight switch can also be used. Once a terminal or wire is found that has power only when the lights are on, attach a 16-gauge wire from there to the switch. Reconnect the battery negative terminal. Now the fog lights will only turn on when the headlights are on. They can be turned on or off as long as the main lights are on. When the headlights are off, so are the fog lights.

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