DOT regulations state that any and all lights on a tractor must be in working order, including lights on a connected trailer.
Below are tips from Phillips Industries on how to get lights working when they malfunction.
Total Lighting Failure: No part of the system appears to be powered.
Possible Causes: Burned out light bulb causing a break in the system; corrosion in harness, light plug or pigtail; ground problem; hydraulic lock from too much dielectric grease; bad spot in wiring harness; issues in the 7-way plug; issues from the tractor.
Check to see if you have power at the 7-way of the trailer. If there is no power there, check the back of the 7-way cable from the tractor. If there is no power there, check the 7-way socket on the tractor.
Check for a good ground.
