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Curt 55515 T-One Connector Trailer Wiring Harness - 4-Flat - Plug-and-Play
CURT 55515 T-One Connector Trailer Wiring Harness with 4-flat trailer end and vehicle-side connectors on white background
 
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: $17.15
Part Number: 55515
Manufacturer: Curt MFG

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CURT Manufacturing 4-Flat Connector Plug-and-Play -- No Splicing CURT 55515 T-One Connector Trailer Wiring Harness

Stop wrestling with butt connectors and electrical tape. The CURT 55515 T-One Connector harness taps directly into your vehicle's factory wiring using OEM-style connectors -- no cutting, no splicing, no guesswork. You get a clean 4-flat trailer end ready to accept any standard flat-4 trailer plug, with a finished look that holds up on the road and through weather.

Plug-and-Play Trailer Lighting -- Done Right the First Time

The CURT T-One series is one of the most widely used vehicle-specific trailer wiring families in the aftermarket. Each harness in the series is engineered for a specific tow vehicle, using connectors that mate directly to the factory tail-light plugs without any modification to the vehicle's OEM wiring. Part number 55515 is a T-One harness with a 4-flat output -- the most common trailer wiring standard for utility trailers, boat trailers, small campers, and cargo trailers.

Unlike universal "splice-in" harnesses, the T-One design preserves your factory wiring integrity. The harness sits inline between the OEM connector and the tail-light assembly, drawing trailer signals cleanly without back-feeding or damaging vehicle electronics. That matters especially on modern vehicles with CAN-bus and load-sensing modules that can throw fault codes when the lighting circuit is improperly tapped.

No Splicing Required

Vehicle-side connectors plug directly into factory tail-light harness sockets. OEM wiring stays intact and uncut.

4-Flat Trailer Output

Standard 4-wire flat connector covers left turn/brake, right turn/brake, tail/running lights, and ground -- fits the vast majority of trailers sold in North America.

Weather-Resistant Construction

Molded connector housings and durable insulated wiring resist moisture, road grime, and vibration for long-term reliability under the bumper.

Compact, Storable Design

At under half a pound and packaged in a 9 x 4 x 3 inch box, the harness coils neatly behind the bumper when no trailer is connected.

What the 4-Flat Standard Covers

The 4-flat connector -- also called the flat-4 or 4-pin flat -- is the industry standard for light-duty trailers. It carries four circuits:

  • Left stop/turn (yellow): Controls left brake and turn signal
  • Right stop/turn (green): Controls right brake and turn signal
  • Tail/running lights (brown): Activates trailer marker and running lights when vehicle lights are on
  • Ground (white): Common ground, typically chassis ground

This wiring set is sufficient for trailers without electric brakes or 12V auxiliary power needs. If your trailer has electric brakes or a breakaway kit, you will need a 5-flat or 7-way connector system instead.

T-One Series Advantage: CURT engineers each T-One harness to a specific vehicle make, model, and year. The result is a harness that fits precisely, routes cleanly, and does not require any adapter or modification at the vehicle side. This is not a universal harness with multiple adapter plugs -- it is purpose-built for its target vehicle.

Typical Installation

Most T-One installations take 15 to 30 minutes with basic hand tools:

  • Remove the tail-light assembly or access panel to expose the factory wiring connector (usually one or two screws or quarter-turn fasteners)
  • Unplug the factory tail-light connector
  • Plug the T-One vehicle-side connector into the factory socket
  • Plug the factory tail light into the pass-through side of the T-One connector
  • Route the 4-flat trailer lead to the hitch area and secure with the included hardware or zip ties
  • Test all circuits with a trailer or 4-flat circuit tester before hitching up
Note on Vehicle Compatibility: The CURT 55515 is vehicle-specific. Always verify your exact year, make, model, and trim before ordering. CURT's vehicle fit guide (available on the product listing or at CURT's website) confirms compatibility. Using a T-One harness on the wrong vehicle will result in connector mismatch.

Common Applications

The 4-flat T-One harness is the right choice when you are towing:

  • Utility trailers and landscape trailers
  • Boat trailers without auxiliary power needs
  • Small cargo and enclosed trailers
  • Motorcycle and ATV trailers
  • Small pop-up campers with no electric brakes
  • Bicycle and sports equipment carriers with lighting
Maintenance Tip: Apply dielectric grease to the 4-flat connector faces before each towing season. This prevents corrosion at the pin contacts -- the most common cause of intermittent trailer lighting failures. Inspect the connector for bent pins or cracked housing annually, especially if the vehicle is stored outdoors.
Need More Circuits? If your trailer has electric brakes, a breakaway system, or needs 12V auxiliary power at the tongue, look at CURT's 5-way flat or 7-way RV blade harnesses for the same vehicle. Upgrading to a 7-way also future-proofs your tow vehicle if you plan to add a heavier trailer later.

Package Dimensions and Weight

Package Length9.0 inches
Package Width4.0 inches
Package Height3.0 inches
Shipping Weight0.43 lbs
UPC612314555154

Connector Specifications

Trailer Connector Type4-flat (4-pin flat)
Circuits ProvidedLeft stop/turn, right stop/turn, tail/running, ground
Vehicle Connection MethodPlug-and-play -- no splicing
Connector HousingMolded, weather-resistant plastic
Wiring InsulationPVC-jacketed, road-rated

Electrical Ratings

Operating Voltage12V DC
Circuit TypeLighting only (no brake controller output)
Brake Control OutputNot included -- lighting circuits only

Product Identification

ManufacturerCURT Manufacturing
Part Number55515
Product SeriesT-One Connector
Harness StyleVehicle-specific, plug-and-play

What's Included

Vehicle-Side ConnectorsYes -- OEM-style, vehicle-specific
4-Flat Trailer LeadYes
Mounting HardwareTypically included (zip ties or bracket)
T-One ModuleYes -- inline diode module or direct-fit connector (vehicle-dependent)
Brake Controller WiringNot included
7-Way AdapterNot included
Important Information
  • This harness is vehicle-specific -- confirm your exact year, make, model, and trim before purchasing. A mismatch will result in connectors that do not fit.
  • The 4-flat connector supports lighting circuits only. It does not provide output for electric trailer brakes or a breakaway battery charger. If your trailer has electric brakes, you need a 5-flat or 7-way system with a separate brake controller.
  • Do not exceed the amperage rating of the factory tail-light circuit. Overloading the circuit with additional trailer loads (LED light bars, winches, etc.) can damage vehicle electronics or blow fuses.
  • On CAN-bus equipped vehicles, verify that the T-One harness for this part number is designed for CAN-bus compatibility. Using a non-compatible harness on a CAN-bus system can trigger dashboard fault codes or cause lighting malfunctions on the tow vehicle.
  • Inspect the connector and wiring before every tow. A damaged or corroded connector can cause trailer lights to fail while on the road, creating a serious safety hazard and potential legal liability.
  • This product is not designed for heavy-duty or commercial trailer applications.


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