Look at this schematic on the bottom picture, it shows how to convert an externally regulated charging system (stock system) to an internally regulated system which is what you have, if you follow that connector on on position 1 on the alternator it ties into the brown wire, that wire routs to the harness connector on the firewall on the driver side. The brown wire on your truck may be routed across the top of the firewall to the passenger side of the engine. Humm maybe it got painted black?? LOL!.
http://67-72chevytrucks.com/vboard/s...09&postcount=9