Does yours have a power sliding rear window? If so check the rear window track for water when it rains. I was having the same issue with my 08 and after sealing the rear window I moved to the 3rd brake light neither fixed the issue. I finally got it stopped by putting a dab of culk on the slider(I don't use the rear sliding window)
My 2008 has had the same problem. First time the dealer replaced the 3rd brake light to the tune of about $400. Argued the second one down to $200. Fixed the problem. Let’s see for how long. Next time I will try to seal myself.