7 Pin Towing Trailer Horsebox Caravan LightTester Check Plug Socket 12v WiringThis 7 Pin Tester is designed to indicatea failure in the wiring on your trailer,caravan or horse-box with a six light LED system.Quick, easy and very simple to us.Just plug the tester in the 7 pin socket or the 7 pin plug of the motorand press the yellow button on the side of the tester.If the wiring is good, all the LEDs light up.