Tractor Trailer 7 Pin Wiring Diagram. I drew this crude diagram to help explain. The trailer wiring diagrams listed below, should help identify any wiring issues you may have with your trailer.
One has flat pin and the other has round pins. This is a basic reference article about trailer and caravan wiring; the plugs and sockets that are commonly in use in Australia, and the pin colour codes that are designed to coordinate proper connections, according to Australian Standards. I found that the white (ground) wire from the four way flat was cut short inside the seven way shell, and only the fat white external.
These plugs are usually round and plug in much like an electrical socket would.
Look at the wiring diagram for your truck and determine the designation given to the ground location for the taillights.
I just went through a hassle with a seven way adapter while doing a side job for a buddy of mine here in western TN. I have everything installed up to the wiring that goes into the power distribution center. Some of the other common styles of connectors use five, six or seven pins.