Oh no. How the heck did I get rust here in the south

I have holes rusted all the way through in the back seat area, right where the seat belts attach to the fender well wall

I noticed that the carpet at the corners by the back door are a little damp and I'm thinking I've got water coming in and settling at the lowest spot (where the rust is).
Do the tin tops have some place(s) obvious where they develop leaks? I looked around and nothing jumped out at me. The weather strip looks good around the door and the rear windows. I wouldn't think it could come in from the tail light area. Any ideas?
Guess it's time for the tin snips, some sheet metal, and the rivet gun :-/