Bodywork and Glass Footwell leaks
Generally, the first sign of trouble with a leaking windscreen is this puddle (above) either appearing on top or underneath from mats/carpets.
I spent a very long time indeed (2 years) trying to work out exactly how it was getting in. I tried silicon (aquatic) sealant in corners, tried sealing between rubber and glass along whole bottom edge, and still water came in if it had been stood out in the rain. Invariably its the moment you pull off the next day that it trickles down. Whats happening is the seal is effectively doing the absolute opposite of what its meant to do and diverting any water directly by capillary action to where you don't want it.
Contrary to popular belief, water doesn't force its way into the corners but through any hairline breaks in the seal from further up the windscreen. It works its way under (often right at the very top of the windscreen) and runs, completely unseen, actually underneath the rubber seal and down into the corners where it has nowhere to go whatsoever until it finally rots the metal (see photo below) and has a route to the inside. If you get several days of rain, it will really build up in there, trickling down on to cab floor. In some cases it goes under carpets so you don't always see it.
You can verify this. If it rains overnight, wipe down A pillars so they're dry. Then carefully lift seal with a knife (vertical part on pillars) and see if its wet under there....
