Plug-in Trailer Harness
Tech article by Viperwolf1 and filed under ElectricalTech 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:
Part | Part # | Cost |
4 flat extension | 48145 | $8.99 |
4 flat car pigtail | 48035 | $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.
Ford |
4-pin | |
Running Lights | Brown | Brown |
Left turn/stop | Yellow/Black | Yellow |
Right turn/stop | Green | Green |
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. Color | Mark as |
White | Yellow-Black |
Brown | Green |
Yellow | Brown |
Green | not 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.
February 1st, 2009 at 12:04 pm
Thanks Viperwolf, much nicer than scotchloks.
June 21st, 2010 at 2:36 pm
You are the the man ViperWolf! Thanks for the info. With your help, I made the harness and piratically rewired my entire trailer. Everything is working awesome now.
September 2nd, 2010 at 11:09 pm
thanks,Viperwolf!!this was very easy…oh shit is that supposed to smoke…kidding….patric kaka broncobill67
September 23rd, 2010 at 11:16 am
saved me so much time. thank you
March 5th, 2011 at 3:50 pm
Your posting is very well done and will save me much trouble. Good job..
Thank you…
April 3rd, 2011 at 3:17 am
Thank you! A very simple and well detailed article to help me with my plans!
May 13th, 2011 at 9:05 pm
Thanks, I love non-invasive mods that can be easily reversed without destroying anything. What a great idea!!!
January 22nd, 2012 at 8:02 pm
Good work there VW1. I have my bumper off and b-4 I put it back on I am gonna do this and thanks,
hc
July 27th, 2012 at 11:03 am
This is exactly what I’ve been looking for. Glad I finally stumbled upon it. Thanks Viper.
March 15th, 2014 at 11:37 pm
Thanks Viper, just used your info to make up my own harness.