Well, the first thing to do is remove
all of the sound deadening material from hell from the floor and tunnel. I had pretty good luck using a heat gun and putty knife. It's sucks, but it has to be done to assess the damage. Actually I've found that the more the tar mat puts up a fight, the better....that means there's actual 'metal' below it.
You also need to remove the backpad and rubber firewall mat....evil things can lurk behind it (Thank you Paul for the replacement firewall section, gonna work perfect!
), and by the looks/area of the rust you've got, I'd say your back window is most likely leaking/loose. If you haven't already, pull out the rust promotion mat from the engine side of the firewall as well..............then show us some more pics of what's
really there