Here is my recent experience to add to the existing threads.
By pulling the dome light out and looking up in there with a flashlight and soaking the area (outside) with a hose, it became clear that water was coming in around the rear washer hose, which would be the common cause.
I took mine to the dealer because I still had a month or so left on the original warranty. They replaced the rear center taillight. No change (no surprise).
Since the dealer had indicated that if that didn't help they would pretty much just be trying to seal it, I sealed the area up myself with silicone sealant which seemed to help at first but after a really heavy rain it was obvious I didn't do a good enough job (it's hard to get the sealer around the back of the tube).
A few more additions of sealer did not help and seemed to make things worse during a couple weeks where we were getting some really heavy rains. I could tell by flexing the area that it was not sealed underneath my patch and capillary action was pulling water in. So I resolved to start over. Pulled all the sealer off and I noticed I could lift the grommet around the hose about 1/4 inch.
I cleaned around the whole area with rubbing alcohol. I then put sealer right under the edge of that grommet and pushed it back down into place. Then did a lighter seal around the hose. I let it cure with the back door open this time.
So far, so good though we have only had moderate rain since then. I think I made a few mistakes. I think by using as much sealer as I did, the flexing of the hose every time I opened the tailgate could have dislodged the seal. Similarly, flexing the hose by closing the tailgate before it was cured may have broken the seal. Not cleaning the area well enough may have contributed as well. And some more permanent sealer (I used the silicone because I had it and I figured I could remove it later if I really needed to) might have worked better.