How to Install a Headlight Relay Harness in Your 2004 GMC Sierra 1500

Position the relay harness in the engine bay. Most harnesses are designed to be mounted near the battery or along the radiator support. Route the two output connectors (the ones that will power the headlights) to the driver and passenger side headlight locations. Then, route the two trigger connectors (the ones that plug into the factory harness) to where the factory connectors now hang. Finally, route the power wire to the battery positive terminal and the ground wire to a clean chassis ground point near the battery.
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