I can actually answer this question with some authority, as I use to drive 60s - 70s 80s full size GMs. My 1980 Coupe De Ville Cadillac had it's windscreen slide down 1cm and it leaked as I drove in rain, My 88 Brouham Deligance Cadillac, never leaked a drop neither did my 77 Pontiac Catalina never leaked 500,000 Kms on it. My 64 Pontiac Convertible leaked because the rear window was bad. I believe American cars do leak less but really this is a w126 issue as my 420 SEL leaked like a son of a *****.
I have the car under a $$$$ car cover for the winter, it's never been winter driven in it's 24 yrs!!

So who wants to help me change the rear window seal??