don't let the lights make you crazy...
they are not fused, the wiring runs from a junction under the front of the trailer directly to each light.
inspect each of the lights, look for corroded ground connections under the body of the lamp. usually a new bulb and new fasteners that securely ground the lamp will fix the problem.
most folks do not try to fix the lamps since they are less than 5 bucks each. replacements are available at most napa or truck/farm stores. the fixtures are made by a company called "grote".
good luck, and do not forget to seal the holes through the skin if you replace the lamps!
you call them ferrets, i call them weasels.