Wiring Diagram for a 7-Way Round Pin Trailer Connector on I do have a handy trailer wiring FAQ article that would be perfect for you. I have included a link to it. I have also included a picture that shows the typical wiring diagram of a 7-Way round pin trailer side connector. As you can see, the 7-way round pin trailer connectors do not use the typical pin functions. Trailer Wiring & Electrical at Tractor Supply Co. TSC has trailer wiring plug adapters, clips, harnesses, connectors, converters. 4 wire and 7 pin wiring kits.