7 Way Plug Diagram. I possibly have an issue in my trailer (it's old), but I wanted to make sure I knew where it was supposed to get power from first. No way would I be behind the truck testing it in reverse. (and didn't think to do so!).
If that's the case, you'll need an adapter so that the two systems can connect. The first wire to run through the plug should be a ground from the frame of the tow vehicle to the frame of your trailer. How to Wire Lights on a Trailer
That diagram is very clear on that.
Trailer plug connector diagrams for electrical towing connectors.
Here is a link to an industry standard pin-out diagram. Standard Trailer Wiring Color Codes And Diagrams PDF From Hopkins. It's available at a trailer supply house online, but I can't find it right now.