So a rat seems to have decided it REALLY wanted in and chewed off half of the rubber seal on the internal "blade" to the vent. Anyone have a line on these for a 66 Coronet? Doesn't seem like anyone makes one and the seal design would be tough to duplicate. I'd prefer to not block it off.

Not sure if your vent is integral to the heater box or not but here is a good place to check for foam and rubber seals.

http://www.detroitmuscletechnologies.com/62-65-b-body-heater-box/

It is the one on the inside.
We were able to find a whole assembly for sale, hopefully it's in good shape.

Those black rubber seals on the 66-70 and 71-74 B body (and E body) fresh air vent doors is "over-molded" onto the steel door. It's not replaceable. I get so many requests for these seals that I buy a dozen or so of these vents every year when I'm at the shows so I can help customers looking for them. 1/2 the ones I get are usually pulled away/delaminated from the steel but they can usually be glued back down. It's pretty rare that they are destroyed or "eaten by rodents".

That must have been you that I sold the vent to yesterday. Small Mopar world. Glad I could help. For chap the vent may not have been pristine but the door was the best one I had.

That's us. Thanks for the help.