That was a common problem for me when I still had the steel tub on my Jeep.

I found that I could avoid the problem by using a twice-a-year preventative maintenance procedure where I did a couple of things: (1) I unscrewed each of the light assembly mounting screws a couple of times. This tended to scrape the rust off the screw threads and the hole in the sheet metal, making for a better electrical ground. (2) I used a small amount of Never-Seize on the screw threads. The metal content in that compound helped get a good electrical contact. Once I started doing this, I never had the problem again.