Given all of the photos and posts on this problem I decided to open up my tail lights today and found much of the same problem as others. IMO the gasket material is not up to the task it needs to perform. It compresses too much and then dirt and water enter under it, mostly between the fiberglass and the gasket, not so much between the gasket and the plastic tail light.
Given that my tail lights were not anything more than damp at the bottom, I simply blew out the sawdust and went about putting silicone on both sides of the gasket and then screwing things back into place.
I realize that the silicone will be hard to remove when a light bulb burns out, but figure that that cheap black gasket will shred first and maybe I'll get lucky and be able to just put things back with a little more silicone. (How's that for lazy?)