Wiring, Lights & Towing Mirrors
Trailer Wiring Color Codes
A convention, not a law. It holds often enough to be useful and fails often enough that you should test.
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Short answer
White is ground, brown is running lights, yellow is left turn/brake, green is right turn/brake. On a 7-way, blue is electric brakes, black is 12V and purple is reverse. This is convention - verify before cutting anything.
The 4-way set
| Color | Function |
|---|---|
| White | Ground |
| Brown | Tail and running lights |
| Yellow | Left turn and brake |
| Green | Right turn and brake |
The 7-way additions
| Color | Function |
|---|---|
| Blue | Electric trailer brakes |
| Black | 12V auxiliary / battery charge |
| Purple | Reverse, or surge brake lockout |
Black and blue are the two most commonly swapped in aftermarket installations. Test before connecting a brake controller output to anything.
What to do when the colors do not match
- Start at the lamps, not the plug. Identify which conductor feeds the left rear lamp, the right rear lamp and the running lights by disconnecting one at a time.
- Find the ground first. It is usually the conductor bolted to the frame. Everything else is referenced to it.
- Label as you go. Masking tape and a pen. Future you will be grateful.
- If more than two conductors are ambiguous, rewire. A new loom costs little and removes a category of problem permanently.
Quick picks
Tap any row to jump to the full write-up. How these are chosen - and what we do and do not claim to know - is published in full on our research methodology page.
| # | Product | Best for | Our read | Price |
|---|---|---|---|---|
| 1 |  CURT 7-Way RV Blade Connector Plug (trailer side) Rewire the plug | Replacing a corroded trailer plug | Screw terminals make colors easy to follow | Check price on Amazon #ad Affiliate link - we publish no price; Amazon shows the current one |
| 2 |  Optronics Submersible Trailer Light Kit Start fresh | Rewiring an older utility trailer | New loom removes the guesswork | Check price on Amazon #ad Affiliate link - we publish no price; Amazon shows the current one |
| 3 |  Blazer LED Submersible Trailer Light Kit For old trailers | Old trailers with tired wiring | Low draw suits tired wiring | Check price on Amazon #ad Affiliate link - we publish no price; Amazon shows the current one |
1. Rewire the plug
CURT 7-Way RV Blade Connector Plug (trailer side)
CURT · Best for replacing a corroded trailer plug
The trailer-side plug for a standard 7-way RV blade connection. Replacing a corroded plug is a twenty-minute job and fixes a surprising share of intermittent light and brake faults - far more often than the fault is actually in the truck.
Published specifications
What works
- Cheap, standard part that cures a lot of intermittent faults
- Screw terminals, so no special crimping tool is needed
- Fits the standard 7-way RV pattern used across the US
What does not
- Only the trailer half - a corroded vehicle socket needs the other part
- Short one-year warranty compared with CURT's structural parts
Skip it if
Not the part you need if your trailer uses a 4-way flat - check which connector you actually have before ordering anything.
#ad Affiliate link - we publish no price; Amazon shows the current one
Raw specifications: CURT’s own page. We earn nothing on that link.
2. Start fresh
Optronics Submersible Trailer Light Kit
Optronics · Best for rewiring an older utility trailer
A complete lighting kit - lamps, wiring, brackets and license plate light - for a trailer under 80 inches wide. Submersible rating is the thing to check: a sealed lamp survives a boat ramp, and a non-sealed one fills with water on the first launch.
We have not sourced a full specification table for this model from the manufacturer’s own site at a stable address. Rather than repeat figures we cannot cite, the assessment above is limited to what the design and the maker’s published description genuinely establish.
What works
- Everything needed in one box, so there is no second trip for a missing bracket
- Submersible lamps suit ramps and wet weather
- Sized for the common under-80-inch trailer class
What does not
- Trailers 80 inches and wider need a different lamp configuration by law
- Supplied wire lengths may be short for a long trailer
Skip it if
Wrong kit for a trailer 80 inches or wider - that class needs additional clearance and identification lamps.
#ad Affiliate link - we publish no price; Amazon shows the current one
Raw specifications: Optronics’s own page. We earn nothing on that link.
3. For old trailers
Blazer LED Submersible Trailer Light Kit
Blazer International · Best for old trailers with tired wiring
LED lamps draw less current than incandescent, which matters on an old trailer with marginal wiring and a tired ground. They also come on instantly, and the cheap failure mode of a sealed LED unit is the whole lamp rather than a bulb.
We have not sourced a full specification table for this model from the manufacturer’s own site at a stable address. Rather than repeat figures we cannot cite, the assessment above is limited to what the design and the maker’s published description genuinely establish.
What works
- Low current draw is kinder to old wiring and weak grounds
- Sealed units with no bulbs to corrode in their sockets
- Instant-on, which gives following traffic slightly more warning
What does not
- A failed LED lamp is replaced whole, not repaired
- Low current can confuse some vehicles' bulb-out monitoring
Skip it if
Not a cure for a bad ground - if your lights dim when you brake, fix the ground strap first or you will blame the new lamps.
#ad Affiliate link - we publish no price; Amazon shows the current one
Key takeaways
- White ground, brown running, yellow left, green right - the four-wire core.
- Blue brakes, black 12V, purple reverse on a 7-way.
- Aftermarket and home-built trailers frequently ignore all of it.
- When in doubt, rewire rather than trace - it is cheap and permanent.
How we sourced this
Every figure on this page came from one of the following, each fetched while the page was written. Where a manufacturer or regulator does not publish a number, we say so rather than estimate it.
Common questions
- Is there an official trailer wiring standard?
- The connector formats are consistent across the US market. We could not find a public authoritative document defining the wire colors in a form worth citing, which is exactly why we tell you to verify rather than presenting the convention as a rule.
- My trailer has all-black wires. What now?
- Identify each conductor at the lamp end by disconnecting one at a time, and label as you go. If more than a couple are ambiguous, rewiring is faster than tracing.
- Does the ground really need its own wire?
- Yes. Relying on the hitch ball as a ground path is a classic cause of dim, flickering and cross-connected lights. The ball is a greased steel-on-steel joint, not a connector.
More in Wiring, Lights & Towing Mirrors
Nearly every trailer electrical fault is corrosion or a bad ground. Knowing which wire does what turns a roadside mystery into a five-minute check.
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Reference
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Found something out of date, or a figure you can source better than we did? Corrections get read first. How we research explains how these pages are put together, and contact is one click away.