If the panel is off in the back look at the metal on the inside of the door with the window up. Look for any rust on the door skin. How old is the paint? If they painted over any rust without removing it, then it will just come through within 2 or 3 years. My car had zero rust underneath the car, but the doors are getting rust bubbles in them which starts from the inside. The inside of my doors have big rust patches. The door seal rubber is hard and cracked and not sealing, which probably caused the rust to start on the doors.

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1987 300E