I threw away the wiring harness on my 1950 Model B when I converted it to 12 volts. The way I have mine wired may not be the best way, but everything works. I am using a GM 1-wire alternator (self-exciting). So here goes:
1. 10 gauge wire runs from back of alternator to the battery post on the starter. Positive terminal of battery also hooks to this post. Negative terminal of battery is grounded to battery box bolt.
2. 10 gauge wire runs from the same starter post to on-off toggle switch in control box.
3. 16 gauge wire runs from opposite terminal on toggle switch to voltmeter (replaced ammeter) to ground.
4. 16 gauge wire runs from same post as voltmeter to + terminal on coil, from - terminal on coil to distributor.
5. 16 gauge wire runs from same post as voltmeter and ignition wire through a fuse holder to a terminal on a pull-push light switch mounted to the left of the toggle switch, and from the other terminal on the light switch to the front lights.
The way it is wired, the toggle switch supplies power to the ignition, the voltmeter, and the lights. This keeps me from accidentally leaving the lights on after I turn the tractor off.
Like I say, this is my home-made wiring scheme. It may be wrong, but the thing starts every time, the alternator charges the battery, the voltmeter shows a positive charge, the lights work, and everything cuts off when I cut off the toggle switch.
Good Luck. Hope this helps. I hope that I have not talked down to you, but I know nothing about automotive electronics. I had to kind of guess how this thing would work. So this is about as technical as I can get.