I agree it's likely the brake light. Mine did the same thing and my truck was soaked on the inside after a big rain. I had to pull the rear seat out, and lift the carpet up to let it all dry out. Here's a picture of where my light goes. You can see where the seal had lost contact with the sheet metal at the top and was just letting water run right in. Needless to say I really sealed it up good when I put it back on.
Here's a pic of my truck drying out. haha.
Rick, Charlotte, NC
2006 F150 XLT 5.4L super cab, 2wd
2008 Kawasaki Concours 1400
1999 Kawasaki Vulcan Classic 1500