More than likely to be the third brake light. Here is a link to a great post about how to fix it. http://www.ramforumz.com/showthread.php?t=87346
I recently tackled the issue, although my solution was different. You can read my reply in that post. Either way, take care of it now before your cabin turns into an aquarium, and smells like one too.
If the link doesn't get you there, search for the post titled "3rd brake light leak fix".