Wiring a Relay-Switched LED Light Bar to a Roof Rack

The layout has two separate legs. A thin trigger leg taps the driver-side parking-light positive, runs to a dash switch, and from there to the relay coil. A heavy power leg runs from the battery, through an inline fuse, into the relay, and out to the light bar, which is grounded on its own. Energizing the coil closes the relay contacts and passes battery voltage to the bar. Because the trigger originates at the parking-light circuit, the whole system is only live when the parking lights are on, which prevents a forgotten switch from draining the battery.

Choose a bar sized and rated for your use. The example unit is 34" wide, 180W, and 13,000LM, built from 60 3W CREE LEDs in a spot/flood combination: the 12 LEDs at each end are 60-degree flood optics and the center LEDs are 30-degree spot optics. Buying a generic bar directly rather than a rebadged brand-name unit avoids a large markup for identical hardware.

Collect the relay, switch, fuse holder and fuses, both wire gauges, all crimp terminals, the Scotchlok, the roof-rack mounting clips, and the sealing supplies before starting. The wiring path itself is straightforward; the fiddliest part is tucking the run neatly under the windshield trim, so plan the route in advance.

Clamp a Scotchlok onto the red positive wire at the driver-side parking light and crimp a length of 16ga wire into it. Route that wire above the fender liner and through the small pass-through hole at the driver's footwell; if the car has no foglights, pop out and discard the clear plastic plug covering that hole. Pull through enough wire to reach the intended switch location, add roughly 4 inches, then cut it and fit a small red-sheath female clip. Label this lead as voltage-in (red).

If the switch is illuminated, cut a second 16ga wire, terminate one end with a red clip for the switch lamp ground, and feed the other end back through the footwell hole into the engine bay, where solid ground points are easier to find; crimp a ground ring and bolt it to bare sheetmetal. Then run a third 16ga wire with a red female clip on each end from the footwell out to the relay area to carry the triggered signal; label it signal-out (white or silver). Connect all three leads to the switch and mount it, most easily in a blank dash plate.

Fix the relay in place with at least a zip tie so it cannot rattle loose. Wire the coil pins, 86 and 87, to the switch signal and to a ground. Feed pin 30 directly from the battery through the inline fuse (sized to the bar's current draw), using a large ground ring on the positive battery-terminal clamp bolt; keep the fuse holder near the relay. Connect pin 85 to the light bar. Use the 10ga wire and yellow clips for these high-current contacts. With only the coil connected, flip the switch and confirm you hear the relay click.

Crimp a pair of male clips onto the light bar's leads and matching female clips onto the relay and ground wires. LEDs conduct in one polarity only, so if the bar stays dark you can simply swap the two clips instead of re-doing a soldered or heat-shrunk joint. These plug connectors also let you unclip and lift off the bar whenever the rack is needed, leaving the rest of the wiring intact for a plug-and-play reinstall.

Bolt the bar to a square-bar rack using standard carriers. The Thule Ride-On Adapter clips place the bolt along the centerline of the load bar, which is preferable to the Yakima SNAR clips that leave the bolt hanging off the front or rear edge. As the nuts are tightened, the carriage bolts tend to spin in the plastic rack clips before they seat, so add a second nut and jam it against the first to lock everything and stop it backing off while driving.

Bring the bar's leads down and tuck them beneath the windshield molding. Saw off the top corner of the trim piece just enough to let the wires drop cleanly into the channel while the trim still sits flush. Run the wires through the roof-rack structure so they cannot flap around or get pinched in a door. Removing this trim for the first time can disturb the weatherstripping, and the corner notch is not a watertight seal, so work carefully.

Turn the parking lights on, flip the switch, and confirm the bar lights. Apply dielectric grease to the exposed connections to resist corrosion. Once everything works, run silicone adhesive along the windshield trim to keep it from flexing out or lifting, especially if any factory retaining clips are broken, then hide the plug connectors near one of the larger clips that secure the window trim.