Or, when changing a trailer light bulb, unplug your harness, or at least turn the lights off. I was changing a fender clearance light this summer as I was about to leave on an early morning, since it was dark I noticed it out and always have a couple of spares lying around. When I pulled the bad light off, the hot lead hit the fender and caused a short and burned out the converter box. If it was the truck, it would only have blown the fuse since it doesn't use a converter. I had to swap trucks in the dark and was late picking everybody up because of it!! And, had to buy a new converter for the Explorer.