Truck Bed LED Lights Wired with a Dome Light Relay
Step-by-step guide made from Indy Farm Life's YouTube video.
Install 12V LED pod lights inside a pickup truck bed and wire them through a 5-pin automotive relay triggered by the cab dome light circuit. This setup automatically illuminates the truck bed whenever a door is opened or the interior dome light is switched on, while preventing battery drain using the vehicle's built-in dome light shutoff timer.
The full step-by-step Spetz Specs for this project (every part, size, diagram and step, free): Truck Bed LED Lights Wired with a Dome Light Relay Spetz Specs.
- Difficulty: beginner
Disclaimer
- This project involves 12V automotive electrical wiring, battery connections, and tapping into factory wiring harnesses. Verify your vehicle's wiring diagram, protect the circuit with an appropriately rated inline fuse, and consult a qualified automotive technician if you are unsure how to perform the wiring safely.
Safety
- Install an appropriately rated inline blade fuse (10A to 15A for LEDs) on the positive lead close to the battery terminal to protect the wiring against short circuits.
- Cap or insulate the unused 87a relay harness wire so the live lead cannot short against metal under the hood.
- Encapsulate underbody wiring in split plastic wire loom and secure it with zip ties away from moving or hot components.
Must know before you build
- Wire LEDs to relay pin 87, not 87a; using 87a turns bed lights on when dome lights turn off.
- Insulate the unused relay lead 87a; it stays live when lights are off and can short to ground.
- Downsize the inline fuse to 10A or 15A; 30A fuses won't protect low-draw 16-gauge LED wiring.
- Tap the dome light wire instead of a tailgate switch so lights shut off with tailgate down.
- Encase underbody wiring in split loom and tie along frame rails, clear of hot exhaust components.
Tools
- Wire Crimper and Cutter (hand wire crimping and cutting tool)
- Soldering Iron (electric soldering iron) (optional)
Materials
- LED Truck Bed Light Strand (1 strand (8 pods), 12V LED pod light strand, 6 LEDs per square, 8 squares per strand)
- 5-Pin Automotive Relay with Pigtail Harness (1, 5-pin SPDT automotive relay with socket pigtail, 5-wire harness (terminals 30, 85, 86, 87, 87a))
- Electrical Wire (2 rolls, low-voltage stranded primary wire, 16-gauge, black and red)
- Electrical Tape (1 roll, vinyl electrical tape, black)
- Butt Connectors (insulated crimp butt splice connectors)
- Male Blade Connectors (1, insulated male spade terminal disconnects, 16-14 AWG, blue)
- T-Tap Wire Connector (1, self-stripping quick-splice T-tap connector, 16-14 AWG)
- Inline Blade Fuse Holder with Fuse (1, inline automotive blade fuse holder, 10A or 15A blade fuse recommended (30A shown))
- Crimp Ring Terminals (insulated ring terminal connectors)
- Corrugated Split Wire Loom (split plastic wire conduit)
- Zip Ties (nylon cable ties)
Step by step
Step 1: Identify the Relay Terminals and Harness Wires
Unplug the 5-pin relay from its pigtail socket to inspect the numbered terminals and identify the battery power, ground, dome light trigger, LED output, and unused 87a wires.
- Separate the relay from the pigtail socket. Pull the 5-pin relay body off the wired pigtail socket and inspect the numbered metal pins inside the relay base that correspond to the wiring diagram.
- Tip: The relay socket is keyed so it can only plug onto the relay pins in one orientation.
- Identify each wire on the 5-pin relay harness. Sort the five wires on the relay pigtail: set aside the yellow center wire (87a) as unused, and identify the red wire for positive battery power, the black wire for negative ground, the blue wire for positive output to the LED lights, and the white wire for the incoming dome light trigger signal.
- Tip: Leave the center yellow wire (terminal 87a) disconnected when wiring a normally-off circuit.
- Common mistake: Don't confuse voltage with amperage when sizing the relay circuit: both circuits run on 12V, and the relay allows a low-amperage trigger circuit to switch a higher-amperage load.
Step 2: Map Out the Relay and Dome Light Wiring Circuit
Plan the normally-off positive-trigger relay circuit by mapping the connections from the battery and inline fuse holder to the relay, dome light T-tap trigger, and 12V LED lights.
- Map the battery, inline fuse, and relay ground connections. Cut the looped wire on the inline fuse holder in half, plan one side to attach to the positive battery terminal with a crimp ring, route the other side of the fuse holder to the red wire (terminal 30) on the relay, and run the black relay wire (terminal 85) back to the negative battery terminal.
- Tip: Cut the looped wire on the inline fuse holder in half and crimp a ring terminal onto the battery-side lead.
- Map the dome light trigger and LED light output circuit. Connect the white trigger wire (terminal 86) from the relay to the T-tap on the truck's dome light wire, run the blue wire (terminal 87) from the relay to the positive lead of the 12V LED lights, and ground the negative lead of the lights.
- Verify the normally-off positive-trigger diagram and isolate terminal 87a. Match the layout to the 'Normally OFF Relay with a POSITIVE Trigger' schematic on the relay instruction sheet, and leave the middle terminal 87a wire disconnected so the bed lights do not stay on when the dome light is off.
- Tip: Use the 'Normally OFF Relay with a POSITIVE Trigger' diagram included with your relay.
- Common mistake: Connecting the LED positive wire to terminal 87a instead of terminal 87 inverts the operation so the bed lights stay on whenever the dome light is off.
Step 3: Wire the Inline Fuse and Relay to the Battery
Connect the inline fuse holder to the positive battery terminal, wire the fuse output to the relay power input, cap the unused 87a relay lead, and attach the ground wires to the negative battery terminal.
- Connect the inline fuse holder and relay to the positive battery post. Attach the input lead of the inline fuse holder to the positive battery terminal, and run the red output wire from the fuse holder up to the relay harness power input.
- Cap the unused 87a wire and land the ground wires on the battery. Wrap electrical tape around the end of the unused center 87a wire on the relay harness to cap it off, and attach the relay and light ground wires to the negative battery terminal with ring terminals.
- Tip: Verify pin numbers on your specific relay harness rather than relying on wire colors alone, as different brands use different colors for terminal 87a.
- Watch out: Cap the unused 87a center wire on the relay harness with electrical tape so it cannot short against metal under the hood.
Step 4: Tap the Dome Light Wire and Route the Trigger Lead
Locate the dome light wire in the harness under the driver-side door step plate, clamp the T-tap connector onto it, and route the trigger wire through an existing firewall opening to the relay.
- Identify the dome light wire for your truck model. Look up the wiring harness diagram for your specific truck year and model on automotive forums, or trace the wire directly from the cab dome light assembly to identify the correct wire color.
- Tip: Check online owner forums for your specific truck year and model to find the exact dome light wire color and harness location, or trace the wire near the dome light itself.
- Clamp the T-tap onto the dome light wire and route through the firewall. Access the wiring harness beneath the driver-side door step plate (or up near the emergency brake), clamp the T-tap connector over the dome light wire, crimp a male blade connector onto the trigger wire and plug it into the back of the T-tap, and route the wire through an existing hole in the firewall to terminal 86 on the relay.
- Tip: Pass the trigger wire through an existing opening in the firewall rather than drilling a new hole.
Step 5: Route the Power Wires to the Truck Bed Lights
Encapsulate the 12V positive and negative wires in split plastic loom, zip-tie them along the underside of the truck from below the firewall to the rear, and fish them up through the bed stake pocket to splice into the LED strand.
- Encapsulate the 12V wires in split loom and zip-tie under the truck. Enclose the 12V positive output wire from the relay and the negative ground wire inside corrugated split plastic wire housing, drop them down below the firewall, and zip-tie the loom along the underside of the truck all the way to the rear bed corner.
- Tip: Encapsulate the 12V positive and negative wires in split plastic housing to protect them from road debris underneath the truck.
- Fish the wires up the bed sidewall and splice to the LED light strand. Mount the 8-pod LED strand around the interior perimeter of the truck bed under the bed rails (three pods along each side and two across the front bulkhead), feed the strand lead through the upper rear bed corner gap into the stake pocket, fish the positive and negative power wires up inside the rear bed sidewall into the same pocket, and join the wires with butt connectors and electrical tape.
- Tip: Fish the wires up inside the rear bed sidewall to the stake pocket so the splice connection stays sheltered from the elements.