I would look at the stem at the very top of the fixture. If it is removable, there should be a small round gasket on the threaded stem, to prevent water from dripping in.
I have seen some electricians forget to install this little gasket"probably because they didn't know what it was for", and water drips down on to the bulb.
I don't know if you have such a fixture, but I thought I'd throw that out there.
Dnk........
