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Trailer Wiring Harness Installation

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How to Install a Trailer Wiring Harness


Today on our trailer, we'll be doing a rewire using the 35' Wishbone 4-way trailer wiring harness with 42" ground wire, from Optronics, part number A35W42B. Now we went ahead and already removed all the old wiring on this trailer before we began this video. So now we're ready to start with a clean slate. To begin with, we're going to go ahead and tape the two ends of the wiring together to make it easier to feed back to the back end of our trailer. We're going to go ahead and work our way through and tape up about every 5' to 6', just to help keep the wiring together, so when we pull it through the holes already in the trailer, it'll make things a little easier. Now we're going to begin up here at the tongue.

We went ahead and we're going to add about a 4' or 5' section or wire loom, part number 459075-1. We're going to use about 6' total. Now that we're got the loom over the end of the wiring, we're going to go ahead and use some electrical tape to help tape it up. On this particular trailer, we're not going to use the wishbone style, as we're going to run both wires down the same side of the driver's side of the trailer, and then take the passenger side and run it across the back, as this was how the trailer was already wired previously, so there's already holes in the trailer for us to route the wiring. Now we also need to find a suitable ground location, where we use a self-tapping screw and the ring terminal already on the wire.

As you can see here, we found a good cross member on the trailer that we'll be able to put a self-tapping screw in to make a good ground. We're also going to be using some 1/2" loom clamps, which are part number A0500. Now we'll go ahead and finish pulling the wires to the back of the trailer. As you will notice, wherever we went through the trailer, we went ahead and used a little electrical tape where the wires went through the trailer to give it a little extra protection, so when the wires move up and down they don't wear through. We also went ahead and put a zip tie on each to help keep the wires from moving too much.

Now that we've got the wires routed to the back of the trailer, we're going to go ahead and pull the green and the brown wire over to the passenger side of the trailer. Now this wiring harness is much longer than we actually need for this trailer, but we're going to go ahead and route the wire around through the center of the cross member in the back, in case if we ever have to change the lights we've already got some longer wiring there. We'll cut the wire to length on the driver's side, split apart the two wires, and strip back the end. Now the lights on this trailer have three wires coming out of them, a red, a black, and a white. The black wire will be connected to the brown wire, which is the running light wire.

The red wire will connect to the yellow or the green, depending on which side of the trailer you're on. And the white wire we'll put to a ground. We'll go ahead and connect the yellow and the brown wire with the red and the black wire on the driver's side. To do this, we'll use some bud connectors, part number 05730-5, and some electrical tape. You'll also notice that we used about 6" of wire loom here, just to give a little extra protection for the wires coming out of the bottom of the lights. Once we connect the bud connectors on the driver's side, we're going to go ahead and slip a loom clamp over the bud connectors and wiring. When we put our self-tapping screw in, we'll go ahead and slip the ring terminal from the white wire, which is the ground wire for this light, over the screws so that we can make our ground here. Now we're ready to go over to the passenger side, and we'll repeat the same process. But on this side we use the green wire to the red and the brown wire to the black, and again the white wire will go to the ground. We use the same thing where we use two bud connectors, a loom clamp, as well as the self-tapping where we use that for the ground. Now that we have all our connections made and the wire zip tied up, as well as clipping off any excess zip ties, now we're ready to go ahead and test our light functions. To do this, we will use a tester. We'll test to make sure that the running lights work, the right turn, the left turn, as well as the brake lights. As you can see here, all the light functions are working properly. And with that, that will conclude our installation of the 35' Wishbone 4-way trailer wiring harness with 42" ground, from Optronics, part number A35W42B.


Questions and Comments about this Video


Frank B.
There is only one brown wire yet you connect it to both lights, How did you do that?
Etrailer Expert
Reply from Chris R.

To connect that single brown wire to both tail light assemblies, you just need to splice into it to create a jumper wire that goes from one side to the other at some point before you reach the lights. A lot of people will use a "wishbone" 4-Way harness instead like # A20WB , which uses two brown wires so you don't have to worry about adding this jumper.

Info for this part was:

Employee Zack K
Video Edited:
Zack K

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