These MaxxHaul testers are designed to test your wiring harness by simply plugging the tester inline between your toe vehicle and trailer, turn on your lights & check the easy to read LED indicator lights. Pin-Point and trace the problem it is on the tow vehicle or the trailer being towed. Simple plug-and read design to allow you to effectively test your trailer lights harness saving you valuable time and money. Durable molded plug construction will handle different weather condition to provide many years of service. Compact design allows you to store in your tool box, glove compartment or anywhere to be ready when you need it.