round baler electronics

Ok I need some help from the electronic farmers. I bought a case/ih 8450 round baler and am wanting to hook up the bale monitor.
It only has 2 wires, a positive and neg. How can I tell which is which.
The two wires are the red and gray/black coming in from the black round thing at the top of the picture.
One goes to a fuse and the other to the on off switch. which is the positive and which is the neg?

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round baler electronics