Boy, what a question. Many of us have torn our hair out over leaks in this area. I'll just give my wxperience and hope that others will chime in. You should also go to the search page and do a search on "leaks" or windshield leaks".

I had a leak on the passenger side where the small drain hole under the rubber trim drained into the same place water from the air intake grill runs. It goes down and drains from holes in the rocker panels under the door. Anyway, the water from this drain hole ran inside the car instead. There was no way to get into the body to fix whatever the problem was s o I had an auto glass shop fill the drain channel under the rubber trim with urethane caulking. (They said not to use silicone). This worked.

Are you sure you have a windshield leak? There are other places for water to get in at that part of the var. For example, any place wire or cables go through the firewall. I've had a leak around the hinge bolts of the front door.
Try the search.