A while back I received and installed Truck Lite Phase 7 LED headlight and passing lights from Discount Fleet Supply. There are plenty of posts about their quality and about installation. I experienced a problem I have not seen on the Forum. The left passing lights did not work. The original light worked fine in the same plug.
I contacted Discount Fleet Supply and explained. They sent out a replacement, no questions asked. They did mention that mine was the first light that anyone had problems with.
I received and installed the replacement. It failed to work. I plugged it into the right side and it worked. I plugged the right side bulb in the left plug. It did not work.
So the left plug is obviously the problem but the stock light worked fine. Then it dawned on me. The stock bulb does not care about the polarity of the plug. But LED's are different technology and maybe they do care about polarity. Then I finally noticed that the ground pin in the plug did match the ground pin from the light. I reversed the pins in the plug and all is good.
The moral of the story is, if you trouble with LED installation do not assume MOCO configures their plugs correctly.