I had the same problem with my R33 Series1. I took the tail light out, which as craved said, is a bit of work. I sat a small heater facing the back of the tail light for bout an hour for the black stuff to soften then very gently puuled the light off. Makesure to take ALL the screws out b4 you start lol. talking from experience. Tehn once I had the light out, I removed all the old sealent which didnt look too healthy, re-sealed it with some new stuff (dont be shy with it) placed the light back in it's place, which wasnt the easiest of jobs either, let it set for a bit and havent had a drop of water in there since.
Good luck with it mate, hope this helps ya out.
-Dave.