You can do a search to find out that this is a very comon problem. The gaskets that are on the back lights leak a little and the condensation occurs. Some have drilled holes in the lights and say that this solves the problem. I don't want to do that, so I swap the gaskets from left to right or get new ones. When you replace them, be careful not to over tighten them. If you do, they seem to leak shortly after replacing them. Good luck.