I live in Illinois where too much salt is used in the winter. I used to work in a rural area driving on gravel country roads.
I have a 97 GTP with a sunroof and rusty rockers 176k miles driven on gravel roads regularly.
I have a 98 GTP with a sunroof and very rusty rockers (they are gone) 115k miles also driven on country gravel roads more regularly.
I also have another 97 GTP no sunroof 174k miles mostly city and highway and the rockers look pretty good. There is some rust but mostly on the pinch weld and where the front fender liner meets the front of the rocker cover.
I would say a combination of gravel eliminating the paint and protection for the metal the salt and the sunroof drainage are likely key factors in rockers rusting.
