I remember back in the day about ten years ago you could buy sheets of that self-adhesive deadening. Car audio places would be best for that I'd imagine! Thanks for the suggestions on products too.

I did actually lift up all the deadening on the affected area in the rear footwell. It's dirty and damp under there, but fortunately not rusty. Lucky escape! I left a heater blower in the car for an hour to fully dry out the area and next I'll be resealing it all.

So I have found the source of the leak for sure. This morning I spent a while cleaning up the inner arch area with a Dremel, and exploring the full extent of the rust. Underneath some crusty, 35 year old sealant I found this absolute gem:





Fortunately, the hole itself is not too big and there's plenty of decent metal around it. It's a little nasty looking but some chopping and welding then resealing and she'll be as good as new (almost).

With my imminent departure from Canadian shores for a while I'm going to (for now) have to stop the rot in its tracks, clean it all up, and get it welded later on. At least the car will be warm and dry for the time I'm away and I know what exactly needs to be done, instead of constantly drying the rear footwell!