Ford also had something similar on Thunderbirds of that year. At the time, I had an uncle in the Midland, TX area. He knew I liked Fords and he said a co-worker had such a new T-bird 4dr. On a trip on I-20, they got into a small sand storm. Could not get the rear vent closed and lots of sand invaded the Bird's interior. Not happy customers!
ONE issue with the Chrysler system! When I got my car in Spring, 1981, I checked everything to make sure it all worked. Car had about 80k miles on it. I carefully moved the rear vent lever and a creaky-linkage sound came from the package tray area. So it worked and I then put the switch lever back to normal. Later, when I tried it, no sound from back there.
Investigating from behind the dash, I came to find a fractured vac nipple and rubber vac hose. Seems the switch body is made of white plastic and is not very robust where the vac hose to the vent attaches. I always figured I'd try to re-glue it, but that has not happened yet. Everything still as I found it.
Just my experiences,
CBODY67