I've found the culprit!

This crusty bit of wire had been put in place of a fusable link, that had been installed inline of the power feed for the headlight switch. The extra resistance this was putting into the circuit when the headlight needed to draw power was what was causing the voltage to drop.

I've bypassed this and have full use of all systems. I will replace this, along with some of the other questionable wiring in due course.

What is a good substitute for these old fusable links? Will regular inline fuses suffice?

I'd also like to say thank you to all who helped try to solve this with me, your help was greatly appreciated, and your knowledge helped me narrow down and ultimately diagnose the problem.

 

Sometimes Benji gets the bear and sometimes the bear gets Benji.

You did a pretty good job finding that , nice work !

The fusible link is USUALLY located where the wire feeding is connected at the starter relay, or the end of the battery positive cable .
Fusible links are there to handle sudden short high demand where a fuse would normally open. They are color coded and usually come with a terminal large enough to fit a 5/16 - 3/8 stud.. You cut out the old open link and just crimp the end of the replacement link to the wire going inside the vehicle.. I am telling you all this because I don't know what color the open link is on your vehicle.
Most auto supply stores sell them. To find the open link , just tug a little at the wires coming off the positive feed and the one that feels like spaghetti is open and you need to purchase that color.link.If you purchase one that is too small it will open when it should not.If you purchase one that is to large, it will not provide the protection your vehicle is required to have.