Installing a trailer light kit involves mounting the rear lamps, grounding the white wire, connecting color-coded circuits, and testing every function.
The order matters more than difficulty: disconnect the battery, mount lamps on the correct sides, route the harness, ground the white wire to bare metal, and test every circuit.
Complete Installation, Step by Step
Follow this sequence to avoid the two most common mistakes: reversed lamps and a bad ground.
- Disconnect vehicle power. Pull the negative battery cable or unplug the tow vehicle’s harness first. Reese Towpower’s manual for under-80-inch combination kits makes this the first instruction.
- Mount the lamps on the correct sides. LH (left) goes on the driver side, RH (right) on the passenger side. On kits with a license window, the left lamp carries it and mounts under the plate bracket. Point side markers outward; wait until the trailer is fully assembled before installing anything.
- Drill the mounting holes. Most rear combo lights need two 1/4-inch holes on 2-inch centers. Under-80-inch kits also call for one 1/4-inch hole per side for amber side markers. Center, mark, drill, then bolt housings tight.
- Route the harness along both sides. Keep the plug end at the trailer tongue and run wires down each frame rail. Secure with supplied clips so nothing hangs, drags, or touches moving parts, sharp edges, or exhaust. TowSmart calls for three clips per side.
- Ground the white wire to clean metal. Use the ring connector and a screw on bare trailer tongue or frame metal. Scrape off paint and rust first. A poor ground is the most common cause of kit failure.
- Connect the circuits and test. Splice each lamp lead to its matching harness wire, reconnect power, and verify every function with the tow vehicle connected.
When correct, tail lights come on with headlights, turn signals flash per side, and brake lights light together. If shopping for a kit, see our tested roundup of utility trailer light kits.
Wiring Colors and Harness Routing
| Wire Color | Function | Where It Runs |
|---|---|---|
| Brown | Tail and running lights | Both sides |
| Yellow | Left stop and turn signal | Driver side |
| Green | Right stop and turn signal | Passenger side |
| White | Ground | Trailer frame or tongue |
Confirm the plug format (4-way flat, 5-way flat, 6-way round, or 7-way round) before buying; a mismatch needs an adapter or converter.
Why Do Trailer Lights Fail After Installation?
Most failures trace back to three preventable causes. Check these in order:
- Reversed lamps or wires. Left and right swapped, or yellow where green should go. Double-check positions before drilling.
- A poor ground. Paint, rust, or a loose screw at the ground point. Wire must bite bare metal.
- Wiring that hangs or rubs. Loose harnesses drag, catch debris, and chafe until shorting. Secure with clips per every manual.
All show up the same way: one function works, one doesn’t, or flickering on bumps.
FAQs
Can I install a trailer light kit without disconnecting the battery?
You shouldn’t. Working live risks a short that can blow a fuse or damage electronics. It takes one minute and costs nothing.
Do I need a converter to install a trailer light kit?
It depends on the tow vehicle’s factory wiring. Common formats include 4-way flat, 5-way flat, 6-way round, and 7-way round. If your vehicle has a 4-way flat plug and the kit matches, you’re ready. Any other combination needs an adapter or converter box—verify before buying.
What does the white wire on a trailer light kit connect to?
The white wire is the ground. Attach it to clean, bare trailer frame or tongue metal with the ring connector and screw. Remove paint or rust first; a poor ground is the most common failure cause.
References & Sources
- Reese Towpower. “Under 80″ Combination Trailer Light Kit Installation Instructions.” Details battery disconnect and 1/4-inch mounting hole specs.
- Harbor Freight. “Trailer Light Kit Owner’s Manual & Safety Instructions.” Confirms wire color coding and LH/RH lamp positions.
- Lowes. “C7280 Trailer Light Kit Installation Instructions.” Covers ground preparation and post-install testing.