The dominant plug for trailer wiring is the flat pin 7-way plug. There are two main standards, RV code and utility code, and then there are three or four variations of these for manufacturers who wanted to be irritatingly different, like Featherlite. Utility code came first, RV code came later.
- Utility code is W-ground, L-brake, R-charge, B-tails, Y-left, G-right, K-reverse.
- RV code is W-ground, L-brake, K-charge wire, Y-reverse, G-tails, R-left, B-right.
The 7-way wire I use at work corresponds to NEITHER of these, so you always have a mess when wiring plugs and sockets. There's no noise issues (it's all DC) but it is irritating. Philips makes cable which matches RV code, but we don't use it.
RV code is also just stupid. They swapped tails and right turn/stop for no reason whatsoever, and in the process assigned red to left when right starts with the same letter and left doesn't. At least in utility code you had gReen and yeLLow correspond to Right and Left. You also had a red charge wire which was more obvious than a black one, although because trailers are stupid, your grounds throughout are white, and power wires are colored (including black.) Sometimes manufacturers use red and black for wires at the batteries, sometimes black and white.
When you change a molded 7-way plug, cut the cable right behind the plug and look at the cut end of the plug. You will see the wiring colors there. They may or may not correspond to the pins they appear to be behind, but you can probe them out with a DVOM on audible continuity mode and very quickly figure out which pin has what color.
Later I will do a blog on all of the trailer connectors I know of. There are 4- and 5-way flat ("SAE"), 5, 6, and 7-pin round, NATO, and a couple of Airstream-specific connectors, including a 9-pin...