I gone through this many times with the Perko lights, it is always corrosion. Even if you measure 12 volts, a corroded connection will drop the voltage under load. Just last week I hope I solved the problem permanently, buy LED sealed lights. They are not that expensive, use less power and are brighter. They should last forever unless they are mechanically damaged. My only problem was that I unhooked the trailer while I was getting parts and when I returned I had a $38 ticket for leaving a detached trailer on the street.