Dennis-
Here's my thoughts, but don't act on them, as I'm only guessing, but can't help myself from wanting to make a reasonable guess.
It's black, and grounded to the protruding metal box on the Wallas central unit where it originates, right?
Since it's black, it is negative*, by convention, and grounded, in this case. 12v, negative ground. (*Old school rules.)
The switch, or whatever it is, at the top of the photo with the two black wires going into it and the white extra insulation has a metal tab sticking out from it, right?
I'm guessing that the first black wire is a ground wire to connect the central panel to the second panel where the switch is located. Thus it is an affirmative or positive ground connection, the ground being of negative polarity.
Notice that the two panels are insulated from each other by adhesive in much of what can be seen.
Check the tab and see if there are any slide marks on it that would indicate the female connector on the black wire has been attached there.
Caution: This is only a guess. A Wallas owner or technician should give you a positive identification and connection verification based on an existing unit or their own knowledge.
Good Luck!
Joe. :teeth :thup