Thank you everyone.. I ended up pulling the selector out and took the face plate off.
The nubbin where the defrost spot was filthy and gummed over with grease and years of farm dust.
Once clean it worked fine.
When the face plate was back on it restricted the full movement of the selector to...
79 C20 A/C equipped.
Blower fan works on all speeds when selector is on vent or heat.
Blower runs only on low when switching to defrost.
Vacuum lines are good. Blower switch is new. I'm not a 100% sure the AC compressor is good but all the wires are hooked up.