Chris, That looks just like the one I have, or had. Have you been to Missouri? It dosen't take that big of hole to make a mess in the floor board. I've seen them as small as 1/4 inch make a water fall, but they are easier to fix............Steve
What you are looking at is a 79-so none are imune. This is a result of water/moisture running down the windshield into the lower corner and getting trapped under some crack or opening in the factory bonding between the fender mount and metal frame. There is a small piece of fiberglass in a triangle kind of shape that stands about an inch tall that rivits in three spots to the metal frame. The fender is then bonded to it in the extreme upper corner. Where this piece is rivited, there is bonding material. Over time it "debonds" and water gets in. The bonding process etches into the factory primer. When it breaks free, it leaves bare metal. Everytime you wash it, or it rains water gets in and starts rusting it away.
It can be fixed by cutting the upper 10-12" of the fender corner off, then peeling it back. I would also pull the door. You can still get patch panels to replace this corner, and or if it is not that bad, a little sand blasting and chemical rust treatment plus some liquid metal or water proof body filler will do the trick. This one here is beyond trying to "fill", it would need total replacement. A good welder could hammer out a piece of sheet metal to the trick.
Weld, patch it in, rust treat, prime, seem seal, then paint. Follow by putting the triangle piece back down with a good amount of sealer between it and the cowl. Now you can bond the upper fender corner back on, and fiberglass/fill it back smooth onto the car. Hang the door and head for the paint shop.
BTW, this car looked great before it was cut open. There was only a little sign of rust from on top, and a massive water leak when it rained. The car always smelled musty.
Jack, you are right. This rear clip is to become the freaky pull behind trailer complete with seat, sun umbrella, cooler, grilled, storage, radio, and anything else I can cram into it.
Yep, when I had my windshield replaced a similar problem (though not as severe) was discovered. Luckily there was a body shop next door with a great welder very familar (he built a C3 custom for Don Johnson) with c3. He cut out the rust and welded in new sheet metal.
You are right about the "lurking" I had no indication anything was amiss.