The tester includes mating electrical connectors which permit the tester to be positioned between the respective plugs of the respective electrical cables running between a towing vehicle and from a trailer. The container for the tester preferably has magnetic strips along its bottom surface to permit temporary attachment to either the tongue of the trailer or the rear of the towing vehicle. The essential features of the tester are limited to only two simple single-pole/double-throw switches, positioned in tandem, and three diodes arranged as a simple "OR" gate in combination with an audio signal device. This simple circuitry permits a single operator to check (a) the integrity of the wiring harness connected to the towing vehicle's lighting circuits, without leaving the driver's seat of the towing vehicle, and (b) the integrity of the trailer lighting with two simple switch adjustments. Means are also provided for checking for a bad ground connection, and an auxiliary power supply can be used with the tester to permit checking of trailer lighting circuits without the presence of a towing vehicle.