What does each wire color do in a 4-pin trailer connector?

According to the diagram: green = right stop & turn, yellow = left stop & turn, brown = tail, license, and side marker lamps, and white = ground (with grounds shown to both the trailer and the vehicle).

Which wire is the ground on a 4-pin trailer harness?

The white wire is the ground; the diagram shows a ground to the trailer and a ground to the vehicle, both using white wire.

Where should amber side marker lights be mounted on a trailer?

The diagram notes the amber side markers should be located as far forward as practical, exclusive of the tongue, per D.O.T. MVSS 108.

When are the three red rear marker lights required?

The diagram states the 3 rear red markers are needed for trailers over 80 inches wide.

Which side of the trailer has the license plate bracket?

The left rear lamp assembly is shown with the license plate bracket.

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4-Pin Flat Trailer Wiring Diagram - Wire Color Codes & Light Functions

Trailer wiring diagram showing color-coded wires for stop, turn, tail, marker, and ground circuits on a trailer frame

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Trailer wiring diagram with color-coded wires (green, yellow, brown, white) routed to rear markers, stop/turn lamps, and grounds.

AI caption April 2011 · Photo 648 of 1799

This document is a wiring schematic for a standard 4-pin (4-way flat) trailer wiring harness, showing a top-down view of a trailer frame with the wire routing and lamp locations. The diagram identifies the function of each of the four wires: the green wire carries the right stop/turn signal to the right rear lamp, the yellow wire carries the left stop/turn signal to the left rear lamp, the brown wire powers the tail, license, and side marker lamps, and the white wire is the ground, with connection points shown both to the trailer frame and to the vehicle. Amber side marker lamps are shown on both sides of the trailer, with a note that they should be located as far forward as practical exclusive of the tongue per D.O.T. MVSS 108. The diagram also shows three red rear marker lamps at the center rear, noting they are required for trailers over 80 inches wide, and indicates that the left rear lamp assembly includes the license plate bracket. This reference is useful for anyone wiring or troubleshooting a basic 4-pin flat trailer connector on a tow vehicle or trailer.

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Frequently asked questions

What does each wire color do in a 4-pin trailer connector?: According to the diagram: green = right stop & turn, yellow = left stop & turn, brown = tail, license, and side marker lamps, and white = ground (with grounds shown to both the trailer and the vehicle).
Which wire is the ground on a 4-pin trailer harness?: The white wire is the ground; the diagram shows a ground to the trailer and a ground to the vehicle, both using white wire.
Where should amber side marker lights be mounted on a trailer?: The diagram notes the amber side markers should be located as far forward as practical, exclusive of the tongue, per D.O.T. MVSS 108.
When are the three red rear marker lights required?: The diagram states the 3 rear red markers are needed for trailers over 80 inches wide.
Which side of the trailer has the license plate bracket?: The left rear lamp assembly is shown with the license plate bracket.