Voltage drop at ballast resistor

I don't understand either.
"most diagrams provided" If you mean what I posted here - those are by Nacho and the wire colors should just be considered generic.
The only one that is likely to be completely correct, is the one in the service manual for your year. If you have, post a picture of the it becuase I don't have '72 FSM in print or digital.
But in the photo above, the harness has been modified - certainly the connectors changed and at least one wire taped off or ? So it looks modified. What else is modified?

"tapped" In this context, its generally understood as a side tap. Often the quick and dirty squeeze type connectors. Maybe Del reads it the same way. Its very confusing.