Shop Nilight 4-Way Flat Trailer Wiring Tester 4 Pin with Bright Indicators Male & Female Double End Design Nickel-Plated Copper Terminals Light Wire Circuit Tester for Turn Signal Tail Lights in high quality, support 2 Years Warranty. This 4-way trailer circuit tester features a LED light for each electrical functions, turn on the lights and check, you can easily read the LEDs and find out which wiring has a problem.