Leaky '89 S15 Cab

I bought the truck in the spring and now am enjoying the rainy season here. This past weekend quite a lot of water entered the cab and when
driving finds its way into the passenger footwells. There is almost no water visible when I get into the cab and it is only after driving a bit that the footwells fill. The water is apparently under the main floor mat and comes out of hiding when agitated.
The question is: does anyone have any experience with this year/model on where the water is entering?
Thanks in advance.
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