I'm not sure if anyone with an OEM LR Trailer Light Harness has done what I've been doing to the LR's I work on with the OEM setup, but as we all know D2's are getting any younger so if you have an OEM trailer harness setup chances are your OEM wiring from the control box down to the trailer plug could be in very bad shape. I've seen the plastic plugs crumble, I've seen them smashed from going off road, and I've seen the wiring get all torn up as well from being poorly routed.

Now to what I've been doing to customers/friends D2's with the OEM setup is very simple and not expensive at all. I remove the RR Tail Light, find the OEM Grey Box, unplug it from the vehicle wiring side, then I disconnect the harness on the trailer plug side. Now you should have the OEM Grey box in your hands. You'll notice the side heading to the trailer plug is the common green, yellow, black, and white. Cut off the 4 wire grey harness connector that goes to the trailer plug side (NOT THE LARGE ONE going to the vehicle wiring). Then go down to either Wal-Mart or any auto parts store and pickup a 4 wire vehicle wiring harness. They'll range anywhere from 6.00 to 15.00 depending the style or if you get one with the built in LED's to test your wiring. Simply splice it in using some 3M Heatshrink Butt Splices, tape it up if you'd like or cover each Butt Splice connector with some Heatshrink as an extra layer of protection. Then just reconnect your vehicle side harness, and route the new harness down to near your hitch. Now you've got a new 4 wire connector (very common) vs the OEM round plug. Some applications will require an adapter, but that's per the application. I've found the flat 4 wire to be pretty much the most commonly used these days.

It's a simple upgrade/fix and it can keep your lights from acting up if your OEM harness is messed up or the plastic plug itself is messed up causing shorts.

 

The seven terminal plug (round) is needed for trailer brakes, or if you are running/charging something electrical in the trailer. Flat four is only good for small trailers. Better to have the seven terminal and use flat four adapter for light duty hauling.

​​​​​​​ Brian.

 

All the trailer brake setup’s I’ve seen on a LR had their own harness, but that isn’t my specialty. I’ll have to look at the guts of the OEM connector, but only 4 wires run down to it. All the trailers I’ve towed had built in mechanical brakes on the tongue and a flat 4 harness. I’ve never hauled a commercial type trailer like a Big Tex so no clue there.

 

Yeah, if you've mainly hauled U-Haul trailers...not a problem...they all have surge brakes (hydraulic). But, most common/run-of-the-mill car/horse/travel trailers (or any trailer over a certain GVW..."can't remember at the moment") have electric braking systems. Since the factory LR trailer harness doesn't have these features included, they would have to be ran separately.

​​​​​​​ Brian.

 

https://discoweb.org/trailerwire/index.htm