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I'm sure this will be going back to the dealer, but looking for an answer here just to understand.

Have a 2020 Atlas Cross Sport SE 2.0T 4motion. 5 or 6 months ago we had a factor hitch, wiring (4 pin), lower valence, etc installed at the dealer. All OEM parts. Of course, I never checked anything until the night before a trip (which would be now). I've read about needing the trailer software activated, or else the lights won't work. I have the opposite problem, all of the light, all of the time. I plugged in my little trailer and the running and brake lights are all on (incandescent lights, not LED). And that's it. Nothing changes with applying brakes, turning lights on or off, hazards, blinker, anything. Took a spin around the block and nothing changed (didn't expect it to). Vehicle also doesn't seem to know it has a trailer hooked up, as it blows up warnings screaming at me as I back up.

So I unplugged the trailer, and of course, the pins on the vehicle are then dead. Have lots of extra harnesses from past installs, trailer builds, etc. Made a jumper that plugs into the trailer, then alligator clips to the vehicle so I can check things with a multimeter. Hook up all the wires as they would be on a standard plug, and all are at the running vehicle voltage (right around 13V). If the ground plus any 1 wire is hooked up, then all outputs are 13V. If only the ground is hooked, the system sees no load and all 3 hot pins go dead. Hook up any 1 of them and after about 2-3 seconds all 3 read 13V again.

Poked around underneath, but there's really nothing to see. Nicely cloth loamed harness disappears into the upper regions of the bumper cover.

Is this an activation thing? Seems odd, like it'd be dead if not activated, not all hot. Anyone seen this before?

Bumper Audio equipment Gas Electrical wiring Cable
 

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I am not a wiring guy, but I can share a story that might (just perhaps) help. Years ago I went to tow with a new dealer installed hitch on my Touareg and the trailer was a braked trailer. I had to use an RV adapter between the trailer and the truck and trailer and when I hooked up-the brakes went on constant with no change to when my foot was on or off the pedal. I later found that there is more than one way for an RV adapter to be wired and mine was wired so that power was constant to the brake pin. I opened the rv adapter up and switched the wiring (the box had instructions this is common enough I guess) and all worked fine. When I read this I can’t help but wonder if something in your plug is wired one way when it should be another…looking at your picture-it looks like your hitch had a four pin plug bolted into the RV base-there is also an optional full size RV plug that can go in this space-so you might undo the screws holding that plug in and drop it out and see if it can rotate around or something-inside is just a round main socket you can put seven round or flat type sockets into…I guess you have a five or four pin on in the same spot.
 

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Here is a really low resolution picture-sorry I tried to grab a web image of what is under that plug held in place by the four screws but wasn’t able to find anything specific, but this is my memory of what it looked like and the various adapters just plug into this…

Watch Product Automotive tire Font Analog watch
 
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