Roll up the window?

Okay... getting serious:  the most common cause is a leaking CHMSL, or 3rd
brake light in non-factory language.  Frequently, the gasket partialls
fails, lets rainwater in, it runs down the back of the cab, and pools on the
floorboard.  Unscrew it, run a bead of silicone around the mating surfaces,
and screw it back down.