[Daves55Sedan]

On '55 Ford sedans, wagons and hardtops (Fairlane and Customline series) the front doors each have a courtesy light switch at the front of the door-jamb.
The power wire comes from a separate terminal on the headlight switch and lands on the courtesy switch. Another wire comes out of the courtesy switch to the dome light.
The courtesy switch contacts are open when the car door is closed and the contacts close (completing the circuit) when the car door is opened.
When I rewired my cars, I ran an extra green ground wire and terminated it at one of the mounting screws for the chrome-plated pot metal dome light housing to insure good grounding integrity.
P.S., most of the Mainline series cars did not have factory installed courtesy lights or switches.