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There's SIX commonly used "standardized" wiring schematics for trailers. (4-pin flat, 5-pin flat, 5-pin round, 6-pin round, 7-pin round, and 7-pin RV) Somewhere along the line, the RV scheme moved from RV's to horse trailers, to general use. RV plugs used a flat prong on the male end plugs in the beginning. The ability to use one common plug evolved into both 7-pin wiring details using the 7-pin plug with ROUND prongs. The two schemes used some common connections, but the brake wire (and one other that escapes me at this moment) swap. Enter Capitalist American Business. Several of the manufacturers of trailer accessories started making "converters" or adapter plugs to make the inter-action between differet wiring schematics work without a problem. The automotive industry has picked their choices. 7-pin and 4-pin flat plugs, both wired to CORRECT specs for their type. Lots of adapters on the market now that swap from 7-pin to 7-pin RV. There are also 7-pin to 6-pin, 7-pin RV to 6-pin, ect.... It's easy to grab one off the shelf and not realize the difference. If you have any one of the 6 types and the truck, trailer, or adapter was another type, you wouldn't have been any better off. At least this way, the plug fits, just have to swap a couple wires. If you still have the package that the adapter came in, there may well be directions to swap 2 wires internally. They may be provided with ring connectors and screws where the other wires are hard wired. That is to accomodate the swap from 7-pin to 7-pin RV. And then there's "random" wiring. Nothing mandates anyone to wire by schematics anyway. Commonly, people will use the center pin of a 7-pin connector for brakes, a ground connection, back up lights, or what-ever they want. It's SUPPOSED to be a constant hot 12v. (To charge break-away battery/power interior lighting/ect) There's always that chance someone wired things "different" from day one. First place I'd go is checking to see if wire pair needs to be swapped.
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