The rear wheel well rust is just an outside result of rust further in. The problem starts because the outer wall of the rear strut tower rusts through. That lets tire spray run inside the quarter panel. It puddles at the rear corner of the wheel well and you get the rust through you see.
You have to fix the problem in the strut tower before you try fixing the rear wheel well rot, or it will just come right back.
He's right, but people miss it because it's not actually part of the strut tower. It's actually a separate panel that's next to the strut tower.
Here's what you see with the tire off, looking up:
Notice that there's a little discoloration, but no apparent problem. The strut tower looks fine. Keep looking:
I had to put the camera right above the brake drum to get this picture. Still looks OK, kind of, but there's something going on at the left of the picture, up at the top of the strut tower.
Here's what's going on:
I had to put the camera between the strut springs to get this picture. See the gray primer? You're not supposed to see that. It's on the inside of the quarter panel. There's supposed to be a metal panel here that's painted with the same undercoat as the other pictures. You don't see it because it's rusted out. How big is that rusted out hole? After I finished breaking out all the loose rust, I get a hole that's just over 8" wide at the widest, and about 4" high at the highest point.
Here's the template I made of the hole:
(All pictures hosted at www.flickr.com)
Until you patch this hole, you're wasting your time going after rust on the back of the wheel well or down in the running boards. This hole is causing the others.