Remember, it was a typical styling feature of cars of that era to have a more slanted/curved rear window configuration. As mentioned, the water pooled in the areas it couldn't drain from. 3M came out with "AlumaLead" metal repair for the GM cars' issues in that area. The popular vinyl tops made it worse, I suspect.
When my parents got their new '72 Newport Royal 4dr sedan, after it had some time on it, I had noticed how much windnoise it had, which was typically around the moldings. I'd already been at work on the '66 Newport to seek to decrease it on that car, so I was becoming handy with 3M Rope Caulk in filling indentions and such. Knowing of the issues that GM was having with their rear window rust (which seemed to happen in just 2 yrs of ownership!), I didn't want the Chrysler to have a similar fate. I filled the gap between the molding and the glass with a carefully applied strip of dumdum, then tooled it all smooth. But I left the side against the body unsealed. Still no rust in that area, so far. It seemed that Chrysler put more paint in that area than GM did, from the ones I saw. Never saw a Chrysler rust in that area, back then.
Just my experiences,
CBODY67