The way that the light is wired, power is supplied to the socket at all times and the HL switch and door switches ground the circuit to make the light come on. We know that there isn't a short in the hot wire because you haven't posted that your car went up in smoke so it got to be in the ground side. The wires go from the dome light to the left and then back behind the quarter window and down to a 2 prong plug either under or behind the back seat on the left side. The ground wire is YELLOW. From there the wires feed under the edge of the door molding forward to a plug behind the left kick panel. The yellow coming out of that plug goes across the under side of the dash and is split. One side to the left door switch, one to the HL switch and one going all the way across the dash to the right door switch. Usually its a pinched wire. Look where the wires are under the rear seat. Look where the sill molding screw at the rear of the floor by the driver's door (it sometimes gets screwed into the wiring harness). Look under the dash. Any installed components there could pinch the wire. Check where the yellow wire feeds thru the little clips that secure it under the dash. They are sharp and may nick the wire and cause it to ground.