For Lmay - I'm not sure about that rain problem, the only thing I know is the AC hose runs across there - do you run the AC when it's raining? Unless the rain water is running down the firewall inside the engine compartment, then following this hose into the passenger side - that could be it? Possibly the plastic vent holes below the windshield are blocked?

Sorry I can't be of more help, I have never looked into this that closely to know where all the possibilites might be.

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Francis E. Abate
Automotive Restoration & Preservation
Sheet metal, trim and upholstery