Originally Posted by 1993 SLE
and the one that I think it would be because i have a similar problem:
your door seals have shrunk
1993 SLE has the most probable cause. Water runs down against the edge of the door seal molding until it reaches a place where the molding has pulled away from the pinchweld opening a path to the interior, the water continues down on the INSIDE edge of the door until it hits the step, where it runs under the carpet and into your rear footwells. If this is your problem, the fix is pretty easy. You pull the molding apart at the bottom seam creating a gap of about an inch or two, then push the molding back where it belongs all along the pinchweld.
Others here have then filled the gap by various means, the best suggestion (sorry I can't remember who offered this) was to find a seal from a junkyard and cut a little piece to fill the gap you just created. Keeps out the dust and noise.