Why cars rust

Cars since the mid-80's have less corrosion problems because they started using galvanized steel in body panels. This was to use thinner metal to save weight (better mileage) and the old method of "let her rust from the inside" wouldn't have outlasted the warranty, all the short-sighted bean counters cared. They had trouble getting paint to stick initially, which is why you see many with flaking paint, which was repeated in the late 90's from changed solvents mandated by EPA. Almost all trim is now plastic, which fails from uV but not corrosion.

What causes rust?

#1 Salt. Besides salted roads, I cringe at the ads in the 70's of a car driving on the beach at the waterline splashing salt water underneath, plus the cars we saw parked on the Fla beaches whose owners forgot about the tide.

#2 Leaks.
From rain or bad heater cores. This usually just rusts the floors since the carpet stays wet. This happens even on the west coast.

#3. Condensation. A car sitting outside, even if covered, can get condensation all over each morning if the surface temperature falls below the air dew point. This is less a problem out west. Even in Seattle when drizzling, the air feels drier than in the East. Cars don't seem to corrode terribly in the NW, despite the rain.

#4. Cheap and minimal paint inside doors and underside. The factory barely sprayed inside the doors in the 60's and almost no paint at the tops or in the crevices. Treat those whenever you have the door panels off. Scrape and use rust converter paints.