Moisture in tail light

If the lens is removable and can be installed in two different orientations, maybe it's installed upside-down, with a weep hole at the top rather than the bottom. If there's caulking around the fixture, check that it isn't blocking any weep holes, too.

I did have to drill drain holes in the taillights of a car I owned some time ago to keep them from filling up with water. If you do put a hole in, leave it uncovered.