AC/heater vacuum mode switch. Try to procure a part number from a local Chev/GMC dealer. Ask them to search it on their national parts locator network for a dealer with NOS (unlikely, but you might get a hit). At least then you'll have a part number for Google.
As a last resort if a replacement can't be found, you can carefully take the switch apart and lap the mating faces with 400 then 600 grit laid over a flat glass pane to assure a plane surface. Be careful with the retaining clip, because it's also the spring that draws the two halves together for a good vacuum seal. Reassemble with a dab of thick (almost dry) moly or graphite grease.
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