Trailers manufactured for the United States market are usually wired so that each side's turn signal and brake light use a single bulb, a system referred to as common lighting. Tow vehicles manufactured for the Asian and European markets are usually wired with turn signals and brake lights that use separate bulbs, a system referred to as independent lighting. Because of this anomaly it is necessary to install a converter box on an independently-wired vehicle intended to tow a common-wired trailer. Converter boxes can be obtained from recreational vehicle dealerships and tow package installers.
Access the rear light clusters of the independently-wired tow vehicle; pickup trucks usually have access ports beneath the rearmost extents of the sidewalls, while in vehicles with enclosed cargo areas it is usually necessary to remove an interior trim panel.
Identify the functions of the various wires in the five-wire harness to the light clusters. There will be a brake wire, a taillight wire and a ground wire shared by both sides, and a left turn signal dedicated to the driver's side and a right turn signal dedicated to the passenger side. No convention exists to which manufacturers subscribe in color coding these wires, but they can be easily identified by removing one wire at a time from the back of the cluster, then running through the functions until one is found to be no longer working; make a list matching wire colors with functions.
Install the trailer converter box according to its manufacturer instructions, mating the color-coded wires from the converter box to the tow vehicle's harness as follows: brown converter wire to vehicle taillights, red converter wire to vehicle brake lights, yellow converter wire to vehicle left turn, green converter wire to vehicle right turn and white converter wire to ground. Use a cable splice to make each join.
Route the wire from the trailer converter box which finishes in the flat four-pin hitch terminal to the outside of the tow vehicle, and attach the hitch terminal close to the tow package. If it is necessary to drill a hole to exit the wiring from the vehicle, fit the hole with a rubber grommet in order to protect the wire from abrasion. Use plastic cable ties to secure the hitch and the wiring where they are not susceptible to accidental damage, then replace any interior panels that were removed in the wiring process.
