I have the orig wiring schematic from GMC - but the charging system was originally designed by American Lafrance, and I can't seem to find any schematics for it.
I had a similar scenario on my dump truck when I was trying to work with the existing wire harness. My advice is Instead of pulling your hair out trying to figure out the existing wire harness with a schematic, it is simpler to just gut the whole thing and rewire from scratch. The GM wiring harness diagram does give you gauge size, so it won't take much effort to get a brand new stock wiring harness.
Besides, removing all the wire will also allow you to assess if any of it could be potentially reused - if not on its original location, then as a scrap of hookup wire in the new harness.
