I've noticed, in my own searching on this subject, that a lot of folks encounter the issue where Dodge didn't equip many of these trucks with proper tow wiring from the connector behind the front tire back. It's my attempt here to provide some instruction on how to add the missing wiring, to a point. I did not want to run my trailer wiring to the OEM plug at the rear of the chassis, as I wanted a distribution box for both 7-pin trailer plugs (in box and bumper.) This would also give me a point for tying into for headache lights and extra bumper lights later down the road. The installation portion is based on my experience, but the details on the wiring harness differences and plugs should help in your own work (I hope.)
Let's start with the basics of the wiring we are dealing with, the differences between non-tow and tow, and what we want to achieve. I am going to assume that you are in the same boat as me and the wiring goes to connector C129 & C129, but not out. So, let’s start at the ”front” of everything, the connectors for the chassis harness , C128 &C129, are missing wires from the "chassis" side of the plug, as the "truck side" is usually complete.
The previous photo I tied into the blue wire as a temporary solution until I got around to this "repair."
In the following image the red arrows show the wires that are missing from our non tow harness, relayed marker lights, trailer brake, and 12V (I tried to color code them accordingly to aid in ID,) in the case of the ground wire (position 3 in C128) differs in that our non-tow wiring is only 16ga, but is 12ga in the trailer wiring.
Next, let’s get a better look at the rear of the chassis harness connectors and splices. The following image shows the rear frame connectors, there is only one missing from our non-tow harness, C331 or C332. I cannot go into much more detail on this as I didn't pick up a donor harness with this connector, and as previously stated my plan was not to run this in my application.
The next hurdle, and why you cannot just re-pin C128/129 then add C331/332 and run the wires along your existing harness and call it a done deal is how the splices are done to get the remaining three wires into C331/332. In order to give you a better idea of the splices, I cut a 3 wire ground splice open to show the way Dodge "welded" the wire together. In this way we can never truly get an exact factory match tow wiring harness (if you want this, you are more ADD then I am,) I would also suggest you cannot add to these "welds" as well.
The splices that are different between the two harnesses are S312 (ground wire splice) has 5 wire for non-tow but 6 in the tow. Next is S313, reverse light splice, has 3 wire non-tow and 4 for the tow. There are two missing splices, S315 and S316, left and right signals. If you are handy with a soldering iron or have a GOOD set of crimpers, you can reproduce these connections anywhere along the harness and run new wire to your OEM style plug or distribution box, or whatever you choose to do. The following image points out the splices (red arrows are missing, black and purple need a wire added.)
Okay, I hope we have a rough idea of what we want to achieve. So, let’s get this done. Removing the harness is probably the easiest way to accomplish what we are trying to do, I had the truck bed off for another project, and I would say that it is the “only” way to do it. I am not going to walk through the harness removal or how to unplug the connectors, it’s fairly straight forward. Besides if you can’t do it, don’t do the following. To begin, you might ask why get a donor harness, well, after going to every parts house in town, and dealer (all of them) I couldn’t find the “pins” I would need to make my own wires. So I searched high and low for a harness with the tow option that was from a long box, extended cab diesel that I could just plug and play, but they don’t end up in a wrecker or being parted out. My next options was to find any of these trucks in my configuration that I could just re-pin and run, well I even had to settle for less, I eventually found “most” of the harness from a shortbox extended cab gas truck.
Gasser connector with tow wiring.
Gasser connection with tow wires beside diesel without tow.
So, with the harnesses out, we need to disassemble them to swap wires over. Inside each plug, C128/129 there is a white “retainer/lock” that comes out to get access to the retainer tabs in the pins. Just put a small screwdriver into the top near the “tabs” and twist or pry the retainer down, then with a pair of needlenose pliers grab it firmly and pull it straight out. Following images should illustrate.
My diesel harness out and ready to repair and mod.
(extremely similar procedure for the other connector)
Now that the retainer is out, you can get a pick or small screwdriver into the “lock” on the pins and gently pry it up so you can pull the pin and wire out the backside of the connector. The rubber seal might be a little tight so using a pick and prying it a little might help in removal.
Prying "lock tab"
Gently pull out wire and pin.
The rubber “blanks” in our non tow harness can be pushed out by using a pick from the inside and pushing against it out.
Now, you have the blank receptacles, and the needed wires, it’s a simple as putting the wires into the right spot and just pushing the pin into its home until the black “locking tab” clicks secure.
Pushing out "plug" in my harness.
I chose not to add the oversize ground wire, I know grounds are probably the most important and overlooked part of wiring. Instead of doing this, I cleaned and replaced the ground from the box to the cab, and added a ground from my distribution box to the frame and box.
The next step in towing our non towing wiring is to add the splices to the wiring harness. I am a fan of soldering and adhesive heat shrink, I suppose with a pro set of crimpers and the right crimp connectors you can achieve the same thing. I will kind of breeze over this as I don’t really know what else to say, I added the splices between the fuel level plug wiring and the “T” to the right taillights. Make sure you stagger the connections as they add bulk to the wiring harness, and too many in the same area will make it harder to put on the split loom.
oem ground splice non-tow harness
random pick of splice S313
pickture of oem marker light splice
pick showing locations of non-tow splice locations *tape at the black loom is approximation of where the tow splices should be*
My version of splice, cut wire, splay ends, intertwine all 3 ends together, solder heatshrink, add a little liquid electrical tape for good measure.
Time for my spin on the wiring. As seen here is my 7 pin distribution box, this is why I didn’t care for the back half of the tow wiring from the donor truck. In hindsight I should have grabbed more wiring as I was short and had to extend the new tow wiring “leg” to the 7 pin box with some extra 12ga & 14ga wire from a trailer wiring loom, not a big deal, the marker light wiring goes from black w/ orange tracer to a trailer color coded brown.
Image of nearly complete harness.
I used normal "cheap" crimp connectors with the plastic base removed and color coded heat shrink *trailer wiring color coded*