Thanks - I'll give the seats and rear cards a thorough scrub and see what we find. The door cards are black and have speaker holes in them, so I was planning on replacing them.
Since I bought the car 31 years ago, they have always had sheepskin covers on them. The seat bottoms were ripped prior to 1991 and the previous owner got the sheepskin covers. I drove the car for 100K miles, then the kids drove it in high school, but it has sat outside for 15 years or so before I began this restoration. So the seats probably have had - at most - 'only' 15-20 years in the burning Colorado sun.
Thanks again for the help sorting this out. These things are always a mystery.