Tech article by Viperwolf1

Many modern vehicles with trailer towing capability have dealer or aftermarket support for trailer harnesses that simply plug in to the vehicle’s factory wiring harness. Early Broncos do not. However the factory Bronco harness has the advantage of simplicity of design. After examining the taillight harness it became clear that a plug-in trailer harness would be a piece of cake to build.

The stock Bronco taillight harness runs down the driver’s side frame rail toward the rear bumper. Just above the rear spring hanger is a harness connector that supplies running lights, brake/turn lights, and backup lights to the rear. This is a 4-pin flat connector just like those used for light duty trailer applications. There is also a taillight grounding bolt within 6” of the connector. Everything needed for a simple trailer hookup (running/turn/brake lights, and ground) is located right there and the factory harness already uses the most common trailer connector.

Parts List:
PartPart #Cost
4 flat extension48145$8.99
4 flat car pigtail48035$3.99
Soldering supplies or crimp connectors
Wiring loom (optional)
I picked up the pigtail and extension at my local Checker Auto parts.

Basically what you’re going to do is plug the 4-pin extension in between the factory connectors with the pigtail attached to the extension. Pretty simple right? Well almost. The only part that requires thought is the color code of the wires and their attachment to the extension. Ford used its own color code in the Bronco harness and standard 4-pin trailer connectors use almost the exact color code.
Running LightsBrownBrown
Left turn/stopYellow/BlackYellow
Right turn/stopGreenGreen
So far so good. Now for a couple problems. The standard 4-pin has a particular pin order in the connectors and the Bronco harness uses a different pin order. The 4-pin has a ground wire in the connector and the Bronco harness doesn’t, it has the backup wire instead. These problems are easily remedied when the pigtail is connected to the extension. When connecting the pigtail you need to match its wire colors/functions with the Bronco colors/functions, not the extension’s. In other words disregard the extension’s wire colors because they’re not in the same order as the Bronco harness order. For the ground wire in the pigtail you will just attach it directly to the Bronco ground point.
On the extension separate the wires.

Label the wires with the Bronco wire order.

4-pin Ext. ColorMark as
Greennot used

Near the mid point cut the white, brown, and yellow extension wires. Leave the green wire intact.

Split the pigtail wires approximately 11” down. Cut the yellow, green, and brown wires so as to leave the white wire about 10” longer than the other 3. Strip the ends of all wires.

Now attach the pigtail colors to the colors you marked on the cut extension wires. You will be joining 3 wires together at each point, both pieces of the cut extension wire and the corresponding pigtail wire. Don’t forget the heat shrink tubing before you solder.

Slide the heat shrink tubing over the joints and heat. Attach a lug terminal to the white wire on the extension.

If you want a professional look add some wire loom.

Unplug the factory connectors and plug the extension in. Attach the ground lug to the ground point and route the pigtail out the back.

That’s all, you’re done. Now you have a trailer harness that requires no modification of the original Bronco harness and you can add or remove it with just one bolt.