If I'm understanding correctly, you want to put a voltmeter in the console, and you want a switched voltage signal to connect it to? If that's the case, then you could just tie into the blue wire that powers the radio. It's switched, and it's already in the console harness. If all your connectors are clean, it "should" give you an accurate reading of system voltage.
Oh, and BTW, the color wiring diagram you posted above is for the 77. Many (most) systems of the 77 are very similar to the 75, but there may be some differences. I normally would start by looking at that 77 color diagram, but to be sure, you should also double check the old style 75 diagram out of the FSM. Especially wire colors, etc.