The dome light works by grounding the white wire that goes to it. The power wire (orange) always has 12 volts to it. I assume you do not have lights under the dash because you didn't mention them.
The dome light turns on by grounding any one of 3 switches. Left drivers door, right front door, or the dash switch. The dash switch grounds the white wire when it is rotated fully counter-clockwise until it "clicks". When you rotate the switch clockwise, the light goes off.
Unless the white wire shorted to ground somewhere, the problem is usually one of the switches. Best I can suggest, at the moment, is unplugging the connector off each switch and see if the light goes out.
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