This test box wiring connections of the trailer socket on the vehicle car and trailer can be checked and monitored a new thing is the visual display of negative conductors of the trailer socket 13polig it must be connected to 3 minus lines the user can also check the connections of the vehicle sockets negative cables 9 and 10 naturally the tester box comes equipped with an approximately 5 meter long cable and 713 pin combination plug for single person use supplied in a practical pla