1984 electrical problem

I recently started having problesm with my dome light in my 1984 lesabre. When the doors are shut and the switch is off, the light stays on. I checked the switch, and it seems to be wired properly. I also checked the door switches, and they are working properly as well. Does anyone know what might cause the light to stay on all the time?

dash dimmer switch turned all the way may cause this.

How do you know the door switched are working? Also the dimmer switch might be sticking.