It isn't so much the weather extreme's it's the states that use tons of salt on the roads in the winter (like Wisconsin). Cars that come from North Dakota (for example) live in more extreme winters than Wisconsin yet, the rust a WHOLE lot less because of less road salt use mainly.
I thought they fixed the fender rust during the recall of '76 when they added the full fender liners. What I mean by fixed is, it took 5 or 6 years before rust through would start on year-round driven cars instead of 1 year. The liners helped keep the crap (i.e. salt) off the inside, especially where the metal is double thickness and salt water gets in and trapped and it's all but impossible to wash out.
That's the reason I would NEVER drive an Aspen (or Dippy, or Mirada, etc) in the winter around here. No matter how much time you spent at the self serve car wash, and things like oil spraying, the car would be destroyed in a couple years or so. I just couldn't do it. There's only so many left and no matter how clean you keep it or repair the rust as it shows up there comes a point where it takes over and repair is futile. Life in Wisconsin.............................................
Just as an example, I've seen vehicles as recent as 2007 models get scrapped at the shop where I work due to excessive structural rust (Friday an '07 Toyota Tundra went to the graveyard with horrible frame rot, the center of the truck was sagged at least 6" down) and as new as 2009's (Chevy and I hate to say,,,,,Dodge/Ram trucks) with rust through starting on the rear wheel openings. Actually, my bosses 2011 F150, which he keeps spotless btw, has rust pinholes in the cab corners already. There is a reason it's called the rust belt (Minnesota, Wisconsin, Illinois, Michigan, etc........).
