I had the same problem with my 92 SLS. After speaking with a mech friend of mine, he explained to me this happened because of condensation buildup. And the fact that there is not enough air getting inside the taillights to dry out the moisture. His suggestion, which worked by the way, was to drill little holes along the bottom of the taillights to let more air in. I'm not talking about going crazy with the hammer drill, but a few small holes strategically placed should allow the moisture to escape, while not letting to much get in. Hope this helps. I now own a 99 STS and do not have that issue.