Did the 1976 duster come stock with interior lights

LOL.
Rotating the light switch adjusts the intensity of the instrument lamps when the parking or headlights are on.
Rotating the light switch until it clicks turns on the dome light (and any others if so equiped) regardless of whether the headlights and parking lights are on or off.

The clicked position is essentially a switch grounding the interior lights.
Its seperate from the other circuits in the headlight switch.