98 Chevy Cab Leak

My 1998 Chevy Silverado has a water leak on the passenger side floor board of the cab coming from the firewall and fan motor areas when it rains really hard and when the truck is in a certain slope position
while it's raining. I have tried everything I know to find the leak and can't seem to pin point it. I was going to have the window re-sealed but the glass guy said that it wasn't the window causing it.
I should let you know it's not from the heater core nor are any of the drain areas clogged. I have read earlier comments from July 2003 post entitled "98 Chevy cab water leaks" which states a screw in the cowl area that could be causing this but for the life of me I can't seem to figure out what their exactly referring to. Does anyone have any thoughts on this? And not to sound dumb but what exactly is the cowl?
Thanks JC
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In my '01 Silverado that had exactly the same symptoms it was the door weatherstriping. Mine needed to be reglued.
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jcronker wrote:

    Had a problem like that on my 98 century. Would only do it if parked up hill in my driveway at my old house. Dealer couldn't find the problem for 6 months. Then finally I was looking at another century with the service writer and we both noticed my car had a small seal missing!
    After replacing it, the problem went away. The service writter was embarrised it took that long for them to figure it out. Charles
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