A boat trailer's lights run on four wires: one ground and three circuits. Here is what each color does, how the harness runs down the trailer, how to keep the lights alive after years of dunking, and what to check when they quit.
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When trailer lights go dark, check the ground first.
4-way flat trailer wire colors
The colors CURT and EZ Loader both publish for the 4-way flat plug, the most common connector on boat trailers.
Wire
Circuit
What it lights on the trailer
White
Ground
Nothing; it is the return path, bolted to the trailer frame at the tongue
Brown
Tail and running lights
Taillights, rear marker lights and the side marker lights
Yellow
Left stop and turn
The left taillight when braking or signaling left
Green
Right stop and turn
The right taillight when braking or signaling right
White is ground, brown is tail and markers, yellow is left, green is right.
The fifth wire is not the same on every trailer
A 5-way flat plug adds a blue wire, and makers use it differently. CURT's 5-way assigns blue to reverse lights. EZ Loader says its surge disc brakes need an actuator with a backup solenoid wired to the tow vehicle's reverse lights, which locks the brakes out so the trailer can back up, while other surge actuators have a manual lockout instead; and EZ Loader's own color chart uses blue for underwater lights on custom trailers. Check your trailer maker's chart before you connect a blue wire to anything.
Trailers with electric brakes use a different plug with a brake wire. EZ Loader warns that unplugging it disables the trailer brakes.
How to rewire a boat trailer
Sketch the wiring first
Draw the trailer and mark every light, where the harness enters at the tongue and where the ground goes. It saves guesswork when the old wiring is off.
Remove the old lights and wiring
Take off the old lights and pull the old harness out of the frame.
Lay out the new harness
Run the new harness from the tongue down both sides of the trailer to the rear, leaving enough at the front to reach the tow vehicle's socket.
Mount the ground wire
Bolt the white wire to the trailer frame at or near the tongue. BoatUS says the ground connections should be bright and clean, free of paint and rust, and secure. Each light also has to be grounded to the trailer, the way its maker's instructions show.
Install the marker lights
Connect the side marker lights to the brown circuit, and ground each one as its instructions show.
Clip the wires to the frame
Support the harness often along the frame. BoatUS names chafing as the second most common trailer light problem, usually where a clamp or a bend has worn through the insulation.
Install the taillights
Yellow to the left taillight, green to the right, brown to both, and each light's ground to the frame.
Test every function
Plug in and check the running lights, brake lights, both turn signals and the four-way flashers before every trip, as BoatUS advises.
If your vehicle has separate brake and turn lights
Some vehicles use separate bulbs for braking and turning, while trailers combine them on the yellow and green wires. EZ Loader says these vehicles need an adapter that converts the three-light system to a two-light one, and CURT sells the same thing as a taillight converter. CURT calls a vehicle-specific wiring harness the best choice for adding a 4-way flat socket, since it plugs in without splicing.
Keeping water out of the lights
Incandescent trailer lights fail at the ramp. When a hot bulb hits cold water, BoatUS says the thermal shock often kills it, and once the lens seals go, water corrodes the sockets too. Unplug the trailer from the tow vehicle in the staging area, a few minutes before launching, so the lights can cool. EZ Loader says to disconnect the harness before the trailer goes in even when the lights are submersible.
The exception is electric brakes: EZ Loader says not to disconnect them before launching, because the brakes stop working. Follow your brake maker's instructions too.
Sealed LED lights, with the whole unit encapsulated in epoxy, avoid most of this, and BoatUS notes that many trailer makers now fit them from the factory. BoatUS also suggests a removable light bar that comes off before the trailer is backed in. Protect the plug contacts and bulb bases from corrosion with dielectric grease or what the light maker specifies; EZ Loader names waterproof grease or petroleum jelly.
Boat trailer lights not working? What to check
Symptom
Likely cause
What to do
Nothing works at all
Often the tow vehicle, not the trailer
Plug a trailer light tester into the vehicle socket first
Lights work sometimes, or change when the headlights come on
Weak ground
Clean the white wire's frame connection to bare metal and tighten it
A turn signal makes the brake lights glow, or the lights are dim
Poor ground
Check the ground at the tongue and at each light
One light out
Burned-out bulb or a corroded socket
Replace the bulb; clean the contacts with fine sandpaper or an eraser and add dielectric grease
One circuit dead all the way back
A broken or chafed wire
Check the wire runs at clamps and bends, then test that wire with a multimeter
Damage around the plug
A harness that dragged or was stepped on
Replace the harness rather than chase the small wires
How to test trailer wiring with a multimeter
BoatUS's method: unplug the trailer from the tow vehicle and unhitch the tongue and safety chains, so nothing is powered and the ground can only run through the wires. Never measure resistance on a live circuit. Set the meter to ohms. Run a long jumper wire with clips from the pin on the trailer plug back to the light, connect it to one meter probe, and touch the other probe to the same color wire at the light. A reading at or near zero means the wire is good. "OPEN" means it is broken somewhere. Repeat for each wire.
Common questions
White. It should be bolted to clean, bare metal on the trailer frame, usually at or near the tongue.
Yellow is left stop and turn, and green is right stop and turn, as seen from the driver's seat.
It powers the taillights and the marker lights, including the side markers, on both sides of the trailer.
It varies. CURT uses it for reverse lights, surge disc brakes use a reverse-light circuit to lock out the brakes when backing up, and EZ Loader's custom trailers use blue for underwater lights. Check your trailer maker's chart.
A weak ground is the usual cause, especially if the lights change when the tow vehicle's headlights are on. Clean and tighten the white wire's frame connection.
Yes, a few minutes before, so hot bulbs can cool before they hit the water. If the trailer has electric brakes, leave them connected, because unplugging disables them.
Specifications come from manufacturers' published documents and can change. Check your engine or boat maker's current manual before you change a setting or fit a part.