If you have an intermittent light try Dynamic service. With the lens out turn the light on. if it does not light tap the bulb lightly with your fingernail. This will test for a broken filimant that sometime makes contact. If that did not fix it wiggle to switch while it is in the on position. This will test for a loose connection in the switch.
Beyond that it is take the light down and make sure the connection to the trailer wiring are tight.
If all that fails I would change that fixture with another one and see if the problem moves with the change or remains at the sink. If it moves the fixture is the problem. If it remains at the sink the trailer wiring is the problem
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles