As you may already know, there are different kinds of moisture and how it
might get inside the headlamp housing. condensation or direct. if it is via
condensation, I think ventilation is important ... which can be the different
approach to what many people think of, which is to apply more sealant.

with direct entry, you'll need to find the seam that has been compromised in
order to patch. the simplistic, ham fisted approach can be effective which is to
apply RTV butyl around the perimeter of the headlight, i.e. along the seam
between the outer lens and the headlight shell.

without having the benefit of inspecting your headlight, I would float the
possibility that the (normally gray) butyl sealant may have been compromised
especially with aftermarket headlights so inspect the seams carefully.

__________________
-- raymond~
47º 34'N 122º 18'W