The floor behind the driver's seat has been getting soaked lately. After a 360 mile trip in sunny conditions I concluded it wasn't a leak in the body. I moved the seat and raised the carpet narrowing it to what appears to be a vent routed under the driver's seat. It would appear something is plugged allowing the water to travel down the duct rather than drain outside properly.

How about some guidance to find the problem?

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1991 350 SDL
114,000 miles