Could be from 3rd brake light/cargo light on cab, common leak point, or drivers side lower windshield valley has rusted thru
1992 may not have a 3rd brake light but should have a cargo light on rear of cab, same thing as far as leaking
Windshields are mounted/sealed to a ridge that runs around the opening, next to the ridge is a valley/trough that runs around the opening, this is where the trim molding clips in
Water gets into this valley and will sit at the lower corners, and if the paint gives out then rust starts and a hole will occur allowing water to run down into the cab on that side
You can pull out the trim and have a look, also pull off the drivers side kick panel to look for signs of water trails running down
And could also be rust holes in the cab floor, pull back floor covering and have a look