Carpoint 12 Volt tester for measuring towbar sockets on a vehicle and towbar plugs on a trailer or caravan. The test kit comes with 2 adapters so that 7- and 13-pole connections can be checked for correct operation. Test the socket from within the vehicle (with a second person) one by one each circuit using the buttons and levers in the car. 

By using a 9 Volt battery, the wiring of the trailer plug can be checked at the push of a button. Without the trailer being connected to the towing vehicle.