Dome light...

The dash light dimmer feature in the headlight switch and the dome lamp are not related.
Turning that knob far enough will toggle a tiny switch built in the headlight switch that will supply a ground to the dome lamp just like the door jamb switches do when you open the door.
I solder a jumper over the dimmer reostat when it fails which gives me bright as possible dash laights no matter where that knob is turned to but, I can still turn it to far right and toggle the dome lamp on.
if the dome lamp was working before and stopped, the problem may be as simple as ablown fuse.