The control unit mounted on the dash (slide for temperature, fan speed control switch and push buttons for air, defrost, vent, etc.) is a Chrysler part, not Chevy. We found it in a repair manual for a 1988 Chrysler front wheel drive car. The part is still available from Chrysler dealers for about $60 - $70. It actually consists of a couple of parts. One is the mounting piece to the dash. The plastic mounting points on this piece often breaks and can be replaced separately. The mounting piece is <$20.
Another piece is the vacuum push button switch. It is around $50 to replace.
Hope this helps.