Small Round plug on the trailer, Large Round socket on your car. Two different sizes, same 7-pin standard, and you're not stripping a wire at the side of the highway. This Jaycar adaptor lead closes that gap: 7 Pin Small Round plug into the trailer's socket, 7 Pin Large Round socket out to your vehicle. About 20 cm of cable between the ends. Plug both in, check brake lights, indicators, and reverse, then get on with your day. If your connector pairing is different, the PA2061–PA2066 range has all six 7 Pin combinations. Match the plug end to your car's socket, the socket end to the trailer's plug. That's the whole job