Thought it was leaking heater core or third brake light, but took the floor may out and pulled the carpet up and the rubber grommet in the middle of the floor board is missing and just looking and the rubber trim piece on the outside windshield wiper cowel is broken and missin in the passenger side. I believe the combination of road water slinging up through that hole and the cowel piece missing is the culprit.

 

Duck tape the missing floor plug! I'm missing the rubber trim on my cowl also... No leaks.