Cowl sealed off......

But, but, you're gonna miss out on the sheer joy of having a winters worth of crap blown all over you when you open that mailbox door on the first warm day of spring....
Car will always be in the garage so I do not think the first opening of the mailbox door would have been a problem.

Doug you have A/C?

I'd be tempted, if "everything else" is gone, to fab some sort of door you can open/ close just for vent.
No a/c....believe the plates are fastened with sheet metal screws, I might just pull the plates out. I will have to stick my head under the dash to see if a door might work......