Trailer Wiring Diagrams | etrailer Trailer Wiring Diagrams. The round pin style is very rare. The RV style 7-way with flat pins (or blades) is very common. It is often found on newer trucks and SUVs that come equipped from the factory with a trailer hitch. 7-Way Round Trailer Connector (Flat Pin) Standard 4-Pole Wiring Harness - For use with vehicles that have adequate4-Way Flat Trailer Connector · 4-Way Flat Vehicle Connector How To Connect 7 Way Trailer & RV Plug - Diagram & Video 7-Way Trailer & RV Plug Diagram. This guide is here to help you. If you can not figure out your wiring even after reading this guide, then contact your LOCAL mechanic or trailer shop for help. If you are local to us, give us a call if you want to schedule a time to have your trailer fixed.