I was just in that area of mine a few hours ago..It looked real clean..there was a tar like substace slathered all over..Preyy sure it is the "factor rust protection" stuff amc used. However looking at your pics Im wondering where water got in at. My guess would be from arount the tail lights or the rear hatch gasket..Either way if you find out Id love to know, cause if it happened to yours theres a good chance it will happen to mine. Murphys law.