are the stock cab lights in two pieces????
have a bottom piece that would screw into the roof, and then the lense to the bottom part(base)???
if so, i would seal them up once and for all the urethane, the same stuff they use on windshields, once that stuff is there, it's there for good.

 

No, the stock lights are one piece. They screw directly into the cab roof with one screw in front and one in back. I had a crack in one of mine a few weeks ago. I slightly loosened all the screws on the other lights. Seems at least part of the problem is due to overtightened screws. I also suspect the expansion/contraction of the roof metal contributes.