Checking the trailer that hadn't been used for a while, found 4 of the 8 bulbs were blown. It seems to me that I changed more bulbs on that thing more than any car. It is a cheapo 8-foot by 4-foot box trailer. One axle, no suspension dampers. I would think when empty it bounces quite a lot and the motion at the back of the trailer breaks the hot filaments. I never replaced the reversing lights, I guess that is only energised when moving slowly. Anyway I went to auto parts store, to replace them with LED festoon bulbs, but had none, so had to replace with incandescent again.