AC/Cig Lighter and Accessory Wiring Question

[DeBattista]

Putting a half apart 1972 C10 Factory Air Truck back together. I was wondering if someone might know where the open terminal on the backside of the orange AC harness is supposed to plug-in I was thinking of utilizing it as a constant 12 V source for a clock I added, however I didn’t want to use it if it already has a factory purpose does anyone know what plugs in there? I have taken some photos and apologize they were at night, but you see the terminal which has a connector on it has an open mail plug inside of it. I have scoured my diagrams and can’t seem to find it anywhere what might go to it.