I saw a mazda3 sedan today was ruested through the trunk lid on both sides, at the brake lights and around the fenders! I hadn't seen one rusted yet and it was horrible.
Yeah, the most common places where it rusts are:
1. rear wheel well lip (the part in front of the wheel)
2. along the rear fender/bumper - it starts in the part toward the wheel well
3. trunk lid - around the brake light and the badge
There are few other spots I know of on the doors.
On mine, so far only area #1 is affected after 6 years.