Well..., I got inside the cowl area today. The good news the debris was less than I thought. The bad news is the driver's side vent opening was rusted. A few things seem strange. One, the rust was part way across the bottom (that makes sense), but it was rusted more severely on the entire vertical edge closest to the steering wheel. That doesn't make sense. The rest of everything is fine. No rust issues.
The other strange thing is my car has the front end pointed downward (slopped driveway, car backed in) and logically the water should have gone out the drain long before it migrated up hill and leaked through the rusted vent access. I say that knowing there was debris under the cowl, but it was far less, and far less compacted than I thought. It should have drained. The nearest windshield drain hole seems too far away to provide the source either. Nothing under the dash indicates that the windshield gasket is leaking.
Anyway..., what a "joy" it is to work under the dash. Welding and grinding is the worst. The blood runs out of your hands, everything falls on you. I had an old mattress pad and cardboard down and when I started to MIG in a patch panel - of course it caught on fire.
I hate welding sheetmetal (at least with a MIG)!!! It warps and pulls away. Clamping was not easy with the limited access. No matter how clean you get the old metal it still isn't clean enough and you blow holes in it (and to think the whole idea is to get rid of the holes). The welds come out looking like knotted rope. I spent more time putting in a "L" shaped patch 3"x4" than I did making and replacing the whole front passenger floorboard.
After that I'm really looking forward to the passenger side - not.
Tom