I had a similiar problem.
What I did was after removing tailights i saw some serious build up of just general dirt and sedament type material built up on the corner of the light(corner meaning where it rounds to the side of the car to a sort of point, where before would puddle up im guessing and work it's way around the lame seal around the tail light). I cleaned this out and it definetly improved the drainage im sure around and off the taillight.
However my foam seal was pretty sorry, after reading i'm probably going to get some of the 3M stuff and seal it up better for good measure.
But up to now, everything has been staying dry. Hope this helps someone..
