I've seen this on my car - the water seems to settle between the lower rubber seal and the bit of trim on top of the threshold trim.

It is clear on my car that the door rubbers are a bit done in - if that isn't apparent on yours perhaps can you see if the fit between metal and rubber is good or not? When you drive the car do you get lots of air rushing into the cabin through the door seal?

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!