What connects to this part of the control box?

[ih8vols]

Trying to figure out the heating on my 1979 C10. Lots of stuff is disconnected from previous owner.

what connects to the switch that selects the AC/heat blow location?

In the picture I attached I don’t see anything that connects to the piece I’m pointing at. I don’t see any missing parts under the dash that aren’t connected either.
Thanks!

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[dvdswan]

If the truck has AC then I don't believe anything. AC is vacuum operated. If its heater/defrost only then there would be a cable. The cable end would connect were you are pointing and the cable jacket would be secured at the other end below your knuckles.