 Must be some water dripping that gets in there somehow. But if it gets through there and gets into that wiring harness, it'll throw all sorts of weird codes because of the body control module hanging out behind there. Give you some false codes in the computer, too. So I would check that before you take it into a shop just to be sure. Now, if that's the only place it's rusted, you should be able to clean it up, repair the wires, and fix it and get back on the road.