Alfa 75 - rear brakes - wear sensor wires - how many does it use?

[AlfaACT]

Re: Alfa 75 3.0 (90/91)

A new wear sensor brake pad has TWO wires (with separate connectors) out of the pad block.

The wiring harness on the car appears to only have provision for one connection.
And only one of the two wires from the pad is connected!

Q. How many wires need to be connected to something?

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Facts:
The wires form one continuous insulated circuit.
(One wire goes in, loops around and comes back out - ie no fancy switches!
How do I know? When I removed the pad, the wires with the plug "fell" out!)
The disk pad material is electrically conductive!

As the disk pad wears the wire will be exposed (insulation is worn off), and eventually cut through, by the disk to trigger the warning.

So:
Are BOTH wires necessary? Possibly not.
Does it matter which wire is connected then? No.

This seems to be counter intuitive; we might expect the circuit to be "broken" (ie opened) when the wire wears through, but it appears that it might actually "close" the circuit instead by earthing back through the disk. Hence only one wire is necessary and it doesn't matter which one is used. Can anyone confirm?

thanks (again),
Peter