My guess is that the black one that is live with the doors shut is connected to the earth side of the interior light. I've not looked at the wiring diagram, but if the door switches make the ground connection you will see 12v on that wire while the doors are shut but ground when they are open. PO bodge indeed, thought he had found a permanent live... I wouldn't waste too much time troubleshooting it if there is an accessible 12v from somewhere else. Just tape up those wires and poke them back on the hole.
As you have no roof lining i would trace the wires back and take them out and install new to suit your needs. I would not tape them up and leave them as it could make problems in the future sods law just after you have fitted the head lining
you taking it to bristol volksfest or whatever they have called it now? if you are i will take a look for you
