I live near Sacramento. Is the d/s vent heavy or do you mean the p/s vent? I thought the d/s was just a plastic piece with that metal door. I haven't tried taking apart the p/s, but you mentioned it was connected to the heater box.
My car had a bunch of leaves in the cowl and water came in through those vents, rusting out the hinges. I thought about trying to restore the metal and put new rivets on it, but it just seems like a lot of work for parts that are still reasonably available.
That's good to know about the fiberglass piece. I tried Goodmark Industries for the sheet metal and they don't have a repro.