Yeah bud, you got a real problem there.
I went throught the same thing and cured it.
This is what I did.
As I was gonna do a full restore on my 73 sb I had the headliner out.
This gave me access to the area where the rust is, (and the foam).
I cut out 2 interior sections, one on each side just below the rear window vents and just above the wheel well hump in a triangle shape.
This exposed the foam of which the is a lot.
I scraped it all out and there was conciderable rust as this foam is like a sponge and holds water. Not such a bright idea from the Germans.
I scraped all the rust to the point of poking holes in the body cuz it was so thin. I wire wheeled it with a brush type wire for my drill and then treated it with POR-15's rust kit.
On the outside I grinded the bodt to really remove more rust damage.
The ideal thing would be to cut out and weld new metal in there but that wasn't an option for me so I just treated it.
We'll see how long it last.
So thats what I did.
If you want your spouse to listen and pay strict attention to every word you say, talk in your sleep.