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Power goes to the dome lamp through an orange wire coming off of the headlamp switch. The door jamb switches make a ground path for the dome lamp. This should be a white wire. Check to see that you are getting 12 volts to your dome lamp first. That will tell you if it is a problem with the power or ground side of the circuit.
Remove the left sill plate and inspect the wire pair passing underneath. It's common for the wires to corrode and sever if their insulation has been compromised by a retaining screw or pinching by the sill plate. Check the inline connector in the left B pillar for evidence of oxidation. While you're there, momentarily ground the white wire at the inline connector to see if the dome lamp illuminates.