They rust out in the phlenum chambers.To properly fix it you need to cut the bulk head of to cut the rust out and panel patch in another or a easier fix is to pull the heater and under dash bolt ons and clean and fiberglass it then paint it with a sealer. Leaves and muck gather around the air vent opening and over a period of years it gets rust holes in it. The only cars I've seen without the problem were cars that had been garaged most of of the time.
buickman
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