I've had quite a few F, M, & J body cars in my past. They didn't survive around here very well. The rust killed the front fenders and lower rear quarters. The interior head lining fell down after a couple of winter/summer cycles, and the end caps around the taillights self destructed after a few years. Many Mopars guys would overlook stuff like that, but the general public wasn't so forgiving. The value of them as a used car dropped like a rock long before the cars were 10 years old. Once the public lost interest, repairing them for anything more then as a cheap beater, sent many to junk yards for pretty simple parts breakage. Even to try to part them out was a pretty sorry adventure. The hard parts that remained good were not desirable for the hard core Mopar guys (no one wanted a 318, 904 lock up trans, or a 8 1/4 with 2:94, 2:76, or worse yet a 2:45 rear gear), and the bodies with screwed up or missing trim pieces made made of unabtainium made them pretty undesirable for guys that might fix up old cars as car shows.
Pretty much everything that was needed for a car to survive as a survivor, didn't last long enough to get them through the amount of time needed for them to be around long enough for the aftermarket to pick up on the the parts that were needed.
It really is too bad. They were pretty good cars during that time period, which really isn't saying much, only a very few select car models have made it though that time frame.
F,M,and J's not created equal. I know the some of those had much worse bumper issues than later M bodies for example. The bumper fills got better too as did end fills. Same on R Bodies. They had some quality issues that were much better on M's. The '88 and '89 M's at the end of their run were pretty darn good. Still 80's cars and there weren't any big three that were much better. M's got more galvanized steel in '85 and pretty darn durable. Only thing missing was EFI and there was plenty of problems with that and many preferred carbs.
