This is a common rust out location. I had an 89 Ramcharger that did the same thing. The rust had spread to over the door frames, and those began to rust too. Whenever you opened a door, it would rain rust down on you and it wasn't going to be long before that truck turned itself into a convertible.
The problem with those old designs was that in those days Chrysler didn't dip the cab in paint and the roof structure is double walled. There was no paint or rust prevention applied inside along the inner seam of the drip rail/windshield pinch seam. The rust started with simple condensation which would cause a rust bubble to form above the windshield, then once a hole developed, rain would do the rest. In some cases a previous owner would fill that cavity between the ceiling and roof with expandable foam, which made the problem worse. This is not an easy fix. Another rust free cab, is the best option. If you go that route, you should consider making a couple of access holes in the ceiling skin, near the windshield and apply a rust preventive.
Ed
