Can you get a picture of the back of the alternator and this ground wire?
I don't have an answer for you because all the descriptions I am finding says your alternator has a wire to the battery positive and a wire to the L terminal. The ground should be direct to the case and not have a wire. If you look at the wiring diagram the ground symbols means there are no wires for the ground, it is direct to the engine block.
It didn't make much sense but I wasn't going to doubt what you are seeing on the vehicle because I assumed it had been hooked up this way the whole time.
Has this vehicle been running with that wire hooked to the negative terminal or have you been having issues?
Images (Click to enlarge)
Monday, February 10th, 2020 AT 6:39 PM