Start at the tow vehicle. Make sure both the trailer and tow vehicle harness is securely plugged in. Check the tow vehicle owner’s manual to locate the proper fuses for the trailer lights. Inspect and replace as needed. If you have confirmed power from the tow vehicle (make sure headlights are on in tow vehicle in order to get trailer clearance lights to illuminate) check that the white ground wire on the trailer plug is making a good connection to the trailer frame. If all of the trailers lights are still not working after confirming power from tow vehicle and a good ground either the trailer’s harness has been compromised and must be replaced or on very large trailers the plug has been disconnected from the trailer’s main harness.