Well first of all which round one. Heres the wiring diagrams showing the pin out for the plug and socket for the most common circle and rectangle trailer connections in use in australia.

6 Hole Trailer Wiring Diagram Diagram Base Website Wiring
Box trailer wiring diagram australia. 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. Get a flat one. And 7 pin large round. It is the 4 pin connector. Wiring diagram for wiring in trailer plugs and sockets. This car is designed not only to travel one location to another but also to carry heavy loads.
If you have a round connector commiserations. In addition to being light it is advised that the connector does not have any power draining attachment. We stock a full range of trailer plugs and sockets as well as trailer plug adapters ideal for when you borrow a trailer that has a different plug to your car socket. It ought not be carrying significant loads during the journey. Trailer wiring diagrams this is a basic reference article about trailer and caravan wiring. This kind of connector is perfect for customer trailers.
7 pin heavy duty trailer wiring diagram manufactured in accordance with the saej560 standard and in accordance with the australian standard as4735 2003 as required for vehicles and trailers over 35 tonnes. There is a very fundamental trailer wiring diagram australia. These connectors are rated at 40a continuous duty. Details for small round 6 pin and 7 pin flat 12 pin and flat 7 pin. This article will be discussing box trailer wiring diagram. I put this here because i had a hard time finding and figuring this out for my trailers.
Box trailer wiring diagram box trailer wiring diagram box trailer wiring diagram australia pj trailer junction box wiring diagram folks comprehend that trailer is a vehicle comprised of very complicated mechanisms.