Right separate all the wires at the interior light.
Connect one piece of wire to a good earth point somewhere on the car, anywhere where the metal is bare and the wire will stay without you having to hold onto it. Connect that to one contact on the bulb.
From the other contact on the bulb run another piece of wire. Touch this wire to each of the wires at the light. When you touch ONE of them your bulb should light. That is your 12 volt supply.
If the bulb lights when touching any other wire then that also somehow has a supply - that is wrong and you'll have to investigate.
If the bulb only lights when in contact with one wire then wrap your test wire around this so that the bulb stays lit. Then disconnect the other test wire from your temporary earth point and, with the doors open, touch it to the wires that you believe are connected to the door switches. The bulb should light each time. As a double check try this again with the doors shut and, if the bulb doesn't light, you have proved the doors switches work.
Now all that you have to do is make sure, when you re-wire the light, that the live wire ONLY is connected to one side of the bulb and the door switches are connected to the other side.
