Lemme do this in drawings 'cause I'm a bit tired and may not be following clearly,
The car has an early system with chrome box and of importance here, the ground contolling regulator.
Schematically something like this.
and I think is what you just did as a test.
View attachment 1715363490
Which yes is pointing toward the regulator.
But I don't understand why your earlier test didn't get the same result.
