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The red wire gets/got its power from somewhere over near the main bulkhead connector, below the master cyl. On Orange's diagram, it's near the top at the far right, labelled "Fuse Link Red". It has/had its own fusible link. You can hook it to anywhere that has very high-power 12V available to it at all times; you could pick it up at the big stud on the alt for example (which is probably the best place really, since that point has the highest voltage available anywhere in the car), or the stud on the starter where the batt cable goes. I'd recommend using #10 or 5.0mm wire, with a "rust" colored fusible link at the end closest to the batt. Solder the new wire to what's left of the original red wire where it's been cut; NOT just crimp it, and above all, NOT a wire nut; and run it NEATLY inside one of the factory harnesses to wherever you decide to hook it to.
Interesting how whoever had their peterpullers in there, left all the AC connectors piled in that one area.
There should be a license required to buy a pair of dykes, and anybody caught with a pair without a license should be sentenced to hard time.
It's the power for the blower high speed. Draws around 30 - 40 amps. "Just" the blower motor. VERY high power. One of the most power-demanding things in the whole car.
Don't ask why; it just IS. Provide it what it wants, that ye may be cool.
Provide it what it wants, that ye may be cool. 
