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.
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