Like lk21joker mention, the taillight gaskets are a common problem
on the 4th gen. Next check the condition of the trunk seal for
tears or being deformed. If the trunk seal appears OK, you might
try using Shin-Etsu grease to condition the seal( You can get it
at the dealer, expensive but last a long time)
You can also order it here.
http://www.handa-accessories.com/s2000-03.html