Is there supposed to be some kind of gasket or seal between the defroster duct and the heater plenum? The two seem to mate well but I do not see a connection between them.

It just sits on top. They must have figured that hot air rises so it would go right up the duct instead of out the small gap.

If it wasn't for that gap I wouldn’t have found a mouse nest in the defroster duct of the cammer. It had the whole riser plugged.

There are seals at the end of the defroster duct where it meets the underside of the dash. They are mostly crumbling to dust. I sealed mine up with 3/8" foam insulation you can get at the hardware store.

I was thinking of getting some 3/8 to 1/2" neoprene rubber sheet to make a gasket. I figure more air going where I want it cant be bad.

My heater gasket kit did have a 1/4" foam gasket for that spot.