This article is from the Fishing bass FAQ, by Bass Rogue with numerous contributions by others.
The lights on a trailer are just an extension of the tow vehicle
lights. The wiring for these lights starts at the tongue of the
trailer with an in-line four-pin plug. This plug has three male pins
and one female pin. The male pins carry the voltages for the lights,
while the female pin provides a ground or return for all circuits.
The color code for the wires at this plug are the same as on the tow
Generally speaking, the ground wire or white wire from the trailer
plug connects to the trailer frame within three feet of the plug.
Often this ground connection is a poor mechanical connection which
leads to frequent problems. Sometimes, the length of the wire bundle
at the plug is such that it puts strain on this ground connection. If
in doubt, redo this connections to ensure a clean, mechanical sound
connection that is protected from the elements. To relieve any strain
on the ground wire, splice in a short length of additional wire and
use a cable clamp to secure the whole wire bundle to the trailer
The wiring color code on most trailers is only good for the first
three feet, the length of the trailer plug pig-tail. After that, the
wiring can be any color. Therefore, it is often necessary to
physically follow the wire instead of following a wire color. On the
other hand, there are really only three wires to troubleshoot and fix.