Simple....Take the tail lights out and replace the seals. If you want to take the cheap way out, use a weather stripping that is a little thicker than the thikness of the seal on the light. Remove the old seals. Put the stripping around the cavity in the body or the light its self. Place the ends together at the bottom, BUT make sure they fit tight agaist eachother.Use one piece as a continuous loop around the surface(DO NOT make repeditive cuts)Hope that helps.
Second Answer / Optional Upgrade Possibility:
I've been working on a solution to the Supra's brake light water-collecting problem. After buying some new taillights I was inspecting them and noticed the design of them is like a water-collecting container! They have several holes (vents maybe?) in the top of each one but no holes in the bottom, like a bucket for holding water. Before I put my new taillights in, I am drilling small holes across the bottom-rear of the taillight housing, in the black plastic part. These should act as drainage holes. I am also going to put duct tape over the holes in the top to prevent the water from draining in. But if some does get in anyway, the bottom holes should hopefully drain it out and down the metal of the body behind the taillight, hopefully on the outside.
The trunk's spare tire area is equipped with holes for a similar drainage-purpose, but they come plugged with big rubber stoppers. I also keep those out to drain any water that may collect... similar principles.