2000 s10 extended cab pickup-water leak

I've been having the same problem with water in my floorboard in my truck. Someone told me they thought it was a vent in the back wall underneath the carpeting. They suggested that the water leak may be coming from that vent. Has anyone had the same problem, and do you know what that particular part is called. I've looked on GM parts website at the schematics and can't find the part listed.
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Answers
If the truck gets wet inside without having moved at all, then you'd look for defects in the rear window gasket.