Trailer wiring mystery

Yes, I have considered the pin issue. The issue is there with both the original and the replacement connector. I've certainly tried as you mention, Jim! Yes Charlie, I can and have used the non led test light, that's how I knew everything was actually working, but that's not the point. I have a bench test, in mind, that I'm going to do this morning. The original connector (that didn't light the tester) connected to my bench battery and see if it lights the tester, then. My next (on vehicle) test is to make up some leads, to connect to the pins inside the connector, the other end inserted into the tester. That will prove if, or not there is a pin issue. John, I already know there is a quality, 0 ohm ground present, and as I've mentioned, the trailer performs as it's supposed to. I'm hoping todays test will reveal that said ground is not reaching the tester. In my mind, nothing else could cause this. Muleboss, my Prodigy controller does light up a small led when no trailer is connected. It works properly and even so would not explain none of the tester functions working. But keep shootin in the dark, still lookin for the silver bullet. I'll let you guys know the result of today's testing.
 
O K, the bench test revealed the issue. Two minor details tripped me up! the first, is that the orientation of the connector is such that the trailer plug inserts upside down. The second detail, is that the guide on the tester, that should insure that it can only be plugged in correctly is near non-existent. If I plug it in upside down and view it with a mirror, it works fine! Boy, do I feel dumb! But in all fairness, who would have thought "A" that the connector would have been designed that way and "2" that the tester could even be plugged in upside down. Well, there had to be a logical explanation and there it is! Operator Error!!! It's too early to celebrate with a cold one but after a while! The good news is that I won't be waking in the middle of the night stewing over it.
 
Wish I had read this sooner. Over the Christmas break I struggled with this issue with my trailer lights. I was using an adapter - 7 round to 4-flat (installed upside down). There is a key on the adapter, but the socket allows it to fully install with the key up or down.
71Vx6PtxQUL._AC_SL1500_.jpg

https://www.autozone.com/trailer-wi...Ra9ZJHN3hBy7I8vTuH0aAt-CEALw_wcB&gclsrc=aw.ds

The right brake light and turn signal worked fine. The left brake light and turn signal would not work at all. The side marker lights would not work at all. After much connector cleaning, light disassembly, bulb checking, trailer ground cleanup, and frustration, I drove it to my destination. It took hours at my destination and got dark. While loading the trailer, I was wishing I had brought my spare magnetic lights to put on the trailer. On a whim, I pulled out the adapter, and tried to turn it over. Low and behold, it plugged in and my marker lights lit up.

Gotta find a way to label that darn adapter.
 
The connector on my truck is the one with the 7 pin and also the 4 pin flat. That same connector fits late model G.M., Ford and Ram pick-ups, it's hard to believe they designed it upside down. No problem with the trailer, the key would not allow it to insert wrong but I had to twist the cord around in an unusual manner. I was just figuring it was because it was cold and stiff. I added material to the key, on that tester, it won't fool me again. It's good to know I'm not the only one, though.
 
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