A hundred million years ago I noticed that horse trailers often stop or turn without the tail lights warning other drivers, so I always gave them lotsa room.
20 years ago on a road trip my wife was driving and a junky old horse trailer happened to be ahead of us. I warned her of my prejudice about horse trailers, told her to watch out and not drive too close, and I zonked out in the passenger seat for a nap.
Zzzzzzzzzzzzz....WHA!?!? I was snapped awake by sudden deceleration, the seatbelt seemingly tightening as my chest pushed into it, and the sound of my wife gasping in surprise. Sho 'nuff, she drove too close, and the horse trailer stopped without brake lights.
Two days ago we bought a horse trailer. It has a six pin connector with an adapter to seven pin. I hooked it up and only the running lights and one brake light worked. When I tried to pull it the electric brakes locked and I scrubbed two 3-foot tracks in his lawn. The owner told my it was because Dodge pickups are wired wrong (whatta sack of baloney!) and it worked fine with his Chevrolet. I So he offers to deliver the trailer. He hooks up to his rig and nothing works at all ! No lights ! Then he goes to unhitch and they turn on... sorta, running lights and one brake light. So he doesn't unhitch and we go for the delivery (the electric brakes are now free) with me driving behind in my truck behind him, hopefully keeping other drivers from being surprised. During the trip the lights flicker off a few times.
The next morning I get to work on making it all work properly. I discover that the trailer ground wire is attached to the pin for right turn. The trailer was getting its ground coincidentally through the hitch - that is why it failed at first and flickered on the trip. I also discover that the socket for the right tail light is cracked and the bulb will not seat against the contacts properly, so the brake/turn never worked. I dunno how the running light worked. The amount of dirt and corrosion packed in it indicates it has been this way for years.
Then I find the wire for the trailer brakes is attached to the pin for aux 12v / trailer battery charging. Any truck wired for this function would activate the brakes... And since his truck was not wired for this It didn't lock them up... And since they weren't wired to the correct pin the never worked at all.
The color code at the connector was not standard, and it changed colors inside the trailer. I had to pull all the bulbs, and use a ohm meter to check the wiring. I couldn't just look at wire color as the blue wire (blue should be electric brakes) turned black at the tail light.
So I got it sorted out. Turn signals, tail lights, brake lights, and trailer brakes all work wired in standard mode to a 7pin mate to my factory Dodge towing package wiring.
But what the hey... This guy had the trailer for years and the lights never worked right . What is the deal with horse trailer owners? Do the just hook up and go without ever checking their lights? Or they just don't care? And why the botched wiring?
Watch out for horse t. Trailers! The tail lights often don't work!