It seems to me that you have a leak somewhere that collects in an unseen area, then condensates on the ceiling. The cowl area of these trucks are prone to rust. Debris falls into the open cowl intake vents in front of the windshield. The debris can get so bad that the entire area can be completely filled, causing rust holes into the cab, under the dash. This could have taken place many years ago, and cleaned out without noticing the rust holes.
Carl
