It could be many things. Take your tail lights out anyway, if they have water in them. Run a bead of silicone on the housing, then re-apply the gasket. Then, run a thick bead of silicone (clear) on the outside gasket, and re-install the tail-lights carefully. Allow the silicone to cure.

Also, check your sunroof drain tubes. make sure they are sealed properly. And if you have an antenna, check the drain hose on that as well.

Click links for more info:
this is how you can seal the tail light on the outside. You should still take them out and seal the gasket to the housing, and outside part of the gasket to the car body.
http://www.team-integra.net/sections...?ArticleID=550

http://www.team-integra.net/sections...?ArticleID=546

http://www.team-integra.net/sections...ArticleID=1021

hope that helps