Instrument Cluster Removal

No you didn't trace it back to a flasher. Explanation starting from a little deeper into wiring... All wires are hot wires. There's a positive side and negitive side to a current path/circle. So the positive side of your battery and your amp gauge have a red wire. The negitive side of you battery and your amp gauge have a black wire. The black wire on the amp gauge is everything on the negitive side of the gauge and current path but it is just as hot as the red wire.
This black wire goes to a weld splice hidden under tape. The branches go to headlight switch, fuse box, ignition switch, etc...
Another black wire ( hot at switch on ) leaves the ignition switch to another weld splice. These branches go to a flasher module, your inst' panel, etc...
I hope you can now read your schematic again with more understanding. If not, The 12 volts to your inst' voltage regulator came from the ign' switch on a black wire.
We all agree that these black wires should have been pink, or red with a tracer, or black with a tracer, and that is found in later models. That isn't how it was done early on. The truth is, Those who are trained in electrical don't need different wire colors at all. The different wire colors are only to dumb it down for the novice ( we all ). Mfgr's continued to dumb it down more and more over the years.