Similar story. My 76 Power Wagon had an oem auxiliary tank behind the seat back, AND it had a sliding back window. The back window would just BARELY weep rainwater. It seemed to be just a trivial inconvenience. What I didn't know is there was some open cell foam rubber in between the rear wall of the tank and the inner wall of the back of the cab. Over time the foam rubber, soaked with water, & corroded pinholes in the gas tank. When I filled the tank, it would leak fuel. When I pulled the tank, the tank was ruined.