My '89 Diplomat Police car came from California. It's unbelievable how clean it is. The underside does not even have any surface rust! The floors and trunk pan look almost new. Location, location, location.
I've seen a lot of older CA cars, that are rusty. It seems the roof around the lower edges rot out, apparently, the roof skin holds the salt from the air towards the lower edges. Any place where there is salt in the air, or on the ground promotes rust.
The best vehicles, rust wise, that I've seen are the inland southwest where they seldom see snow, or the extreme inner north west where they see so much snow they don't bother spreading salt, and there is no salt in the air. .