I have a similar situation with out 2007 Mazda 6 to the one referring to rust in a Mitsubishi Lancer (Carsguide, August 17, 2012). Our car is rusting really badly at the rear door hinges and into the roof. Mazda sent someone out who gave it a cursory glance and then sent a paint specialist out after I persisted. They are saying that it is environmental rust and that it was not repaired properly after the car was in a minor accident where the base of the boot lid got damaged. I am getting nowhere with Mazda, they take ages to respond to my questions and I am now considering going further with the Fair Trading people. Can you please advise me on whom I can approach?
You need to have an independent body repair person evaluate your car with the objective of determining the reason it's rusting. Graeme Cuthbert is one specialist who assists owners in resolving issues like yours. He can be contacted on 0422 444 335.