Everything that goes through the firewall is a potential source: windshield wiper motor shaft, wiring bundles, throttle cable, etc.

Also, a common problem on mid-80's wagons was the cargo panel windows leaked badly once the caulk dried up. This water rund down the "C" pillar under the rear bench seat and comes out at the edges of the bench onto the floor. Also gets down into the wheel arches somehow and rots out the rear end of the rocker panels.

Believe me, you don't want this repair!

Lift the rubber gasket along the front lower corner of each panel window from the inside and trace rearwards, checking for rust under the gasket. A flexible 1.5" wide putty knife is handy for doing this.

Window will need to come out in order to make repairs and to reseal.