Water leak rear floor (pan) drivers side

Driving me crazy. What could cause this. All doors seals solid. Cleaned the under windshield drain system. Inspected underneath for missing plugs. Had brand new 1986 Grand Marquis and same thing. The mechanics at the dealership knew exact cause and fixed immediately under warranty. Issue on this car started two years ago. Any old time Ford/Lincoln, Mercury mechanics that would know root cause?