the interior light works on the principle of there always being a +ve 12V feed to the light. then there is one wire going to the door open switches. these switches switch to ground when the door is open and so the light comes on as long (providing switch set to "DOOR")
then to override this the "ON" position gives a -VE from the car body.
so the 2 wires going to the interior are; the +ve feed (constant) and the ground for the door open switch
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