Mark, although I guess anything is possible, it seems like it would be unlikely that the windshield seal itselfm could leak, given that it is bedded to the windshield frame in a thick rubbery bedding. When I bought my rig (it was a pre-owned unit), the carpet under the driver's left foot would be wet after a heavy rain. I was looking for a leaky windshield seal too, but found out that the outside marker light on the driver side allowed water in behind the cap, and it ran down the windshield pillar and dripped onto the carpet.
I removed and sealed all the marker lights and have had no further problems for 2 years. Also, when I was at factory a year ago for other work, I had them pressure test the rig to check for leaks, and all they found was a minor leak on the upper radius of the window on the driver's side, which they fixed N/C.
I'm wondering if you might have a similar problem. If it really is the windshield, the watertight seal is between the glass and the frame; not the outer rubber gasket. That is just decorative, to give the windshield a "finished" look, not provide a water-tight seal.
USAF 1965-1971; USCG Master
2002 Horizon 36LD - 2000 GMC Sierra 1500 4x4
Indian River, De