Post #5 diagram is confusing. The field [-] is the brush and brushholder. It has a screw that holds in onto the alternator housing. This is your ground.
The field [+] is the other brush and brushholder. It is isolated from the alternator housing with special washers. Unplug the green wire from the alternator. Grab your test light [an incandescent bulb type with a 4' wire and an alligator clip] and push it into the green wire connector. Mount the alligator clip onto the alternator housing. Check for safety. Turn ign. key to on. The light should go on.