Ford Think lone wire to battery?

Guys, need help to figure out where this lone wire hooks up to on the battery terminals. When I pulled some batteries, I didn’t mark where this one goes. I am thinking on the 72V + battery terminal, but not sure and don’t want to fry what it’s hooked up to.

The manual shows a faint wire but not even sure if this is correct. Any thoughts or can someone remove their seat bottoms to see where it hooks up to?

Ok, I see on the tag in the picture, it says “battery 1 Neg.”

But which is battery 1?

I think I found it in the Service Manual.