C-bodies seem to have endemic rust in the bottom rear window corners and near the trunk hinges. Has the cause of this ever been discovered? Is it just a bad design and an area where water tends to sit? How hard a repair is this?
A bad design. They leaked even when new.
The mid/late 60's was when engineers were moving away from rear window gaskets to Butyl and then Urethane. Add the slope of the trendy "fastback" roof lines, and poor drainage you get rust after a few years. Vinyl top cars get hit even harder since the factory did not even bother painting the roof and rear window area being covered to save money.
Remember when the cars were new,they were slammed out the door quickly as possible and were never intended to last 10,20,or 50 years later.