don’t lose your cool, but do open your ’49 shop manual to page 201, which shows the “Circuit Diagram” of the electrical system. The dome light is powered by a “hot” (always has current in it) wire, labeled 14RXT, going to it from Terminal 5 of the headlight switch. The wire passes through a large multi-pronged male/female connection under the dash that connects to most wires going to the back of the car. In a ’49, this large plug is held to the inner firewall (under the dash) by a clip at the upper left of the inner firewall (directly in back of the hand brake mechanism). From the connecting plug, the dome light wire runs up the driver’s door post and under the headliner to the light, which is not grounded, and then to the dome light switch. When you turn on the dome light switch it grounds itself, completing the circuit and the dome light goes on.
The dome light circuit can be checked by removing the 2 screws holding on the dome light switch and its chrome bezel. Then, pull the switch with its male connection off the wire behind it which has a female connection plug. Then, attach a test light’s alligator connection to any metal ground point and poke the needle end of the test light into the female connection on the wire. The test light should illuminate; if not, that means current isn’t getting to that point and the dome light won’t work. This could be due to a problem with the large connecting plug under the dash, or the wire is broken somewhere along its length or it has become disconnected from the headlight switch.
Note that the front door opening switches do NOT operated the dome light switch on any 2 or 4-door ’49 Cadillac. An exception is that the front door switches will operate the “corner lamps” at the corner of the rear of the roof and the body on a Coupe De Ville. On the other hand, the rear doors on a 4-door sedan will operate the dome light switch
