A trailer hitch is a two-part connector in the wiring harness between a tow vehicle and a trailer; it can be attached and detached as required to connect and disconnect the vehicle and the trailer. The outlet, or female socket, is conventionally fitted to the tow vehicle, and its matched male plug is conventionally fitted to the wiring loom of the trailer. All the wiring is drawn together by the trailer manufacturer into a loom, colloquially called a pigtail, at the tongue of the trailer, and the plug is wired to its end. Depending on the complexity of the trailer's provisions, either a flat four-pin or a round seven-pin plug is used.
Locate the trailer pigtail beneath the trailer tongue. It will be either a bundle of four or seven wires, usually bound together by spiral trunking or black electrical tape and held in place with plastic zip ties. If the trailer is equipped with electric brakes the pigtail will have seven wires, and if not it will have four wires. Purchase a trailer hitch plug accordingly.
Remove four inches of the spiral trunking or electrical tape from the pigtail by winding it backward down the loom and then cutting off the excess with scissors. Be careful not to damage the colour-coded insulation around each of the individual wires.
Use wire strippers to remove one inch of colour-coded insulation from each individual wire. Twist the bared strands of each individual wire between your fingers until you have seven rope-like braids.
Attach a flat four-pin trailer hitch plug by matching the colour-coded wires from the pigtail to their mates on the plug; white to white, black to black, brown to brown and yellow to yellow. Solder the connections, and insulate with a shrink-wrap kit.
Attach a round seven-pin trailer hitch plug by using a screwdriver to loosen the fastener holding the outer case of the plug to its inner terminal hub, and then removing the case. Slide the case onto the pigtail, smaller hole first. Mate the colour-coded wires to the numbered terminals as follows: white to terminal one; brown to terminal two; yellow to terminal three; green to terminal four; blue to terminal five; black to terminal six; purple to terminal seven. Use the screwdriver to tighten the terminal securing screws down onto the wire braids. Slide the case back up the pigtail onto the terminal hub and tighten the fastener.
Before towing the trailer for the first time, and intermittently thereafter, have an assistant help you to test the function of all the trailer lights.