One wire not identified in the wiring diagrams, it’s a black wire with a ring terminal, it runs along with the red/blue and brown wires that go to the starter solenoid. Guessing this black wire might go to frame ground?
That's too big to be a ground. It may be a main power feed that hooks to the battery side of the starter solenoid. Plug it in to the body harness first, then attach a meter lead to it for a continuity check. Then use the other lead and probe around the starter switch and fusebox and see if you get your meter to "chime". Remember to have the battery DISCONNECTED during this test and you can try it with the keyswitch on or off.