Mystery wire on 66 C10 with delux heater and AC

[spudder]

GM vehicles used a brown wire for power to the the heater. The heater should have a dedicated fuse at the fuse block where the brown wire picks up it's power.
The wire you have pictured is for a cigarette lighter. Lighters of that era had a pin on the the receptacle that the wire end pushed onto. Lighters were powered form a "battery fused" source on the fuse block. This provided constant power to the lighter even when the ignition was off.