It's a good point as the project I have, has rusted in practically the same location on both sides.

In this case the sunroof was leaking and I think the water has run to the back via the drain pipes, it's a crazy design, they should have run them straight to the ground

Also, I think the water in the offside floor pan caused it to rust so quickly.

To be honest though, given the age, it's inevitable for a car (in my case, at least) that has been used daily and in all weather.

Martin