So I came across this a few months ago, which is luckily the only bad rust on the car, granted it looks pretty bad, I got a mate who's a engineer/welder he builds custom trikes and that so knows what hes talking about n he says he can help my with it, Its the bottom of the c pillar I think that's what it is correct me if I'm wrong. If anyone's had this problem and how you fixed it let me know.