Grab a piece of paper and a pencil for note taking, then ground the tester by attaching the alligator clip to the vehicle’s frame. Now insert the probe end of the tester into each wire in the vehicle’s trailer wiring harness. Make sure the headlights are on, and test each blinker and brake light. When you test each wire, a light on the device should indicate a complete or broken circuit. Write down the colored wire that controls each light (right blinker, left blinker, brakes, etc.). The wire colors on your vehicle’s connector should correspond to the connector used for your trailer.