I think I saw an earlier post about the tach not reading what it should. But your question about the path of the red-white wire -- hmmm -- I didn't then and don't know now for your car -- figured someone would respond from knowledge.

So here's my take on it. In my 79, the earlier dash and cluster, the wire did not exist ready and waiting for my newly installed tach. I had to run it myself, and being lazy and always averse to piercing the firewall unnecessarily, I tracked the coil wiring to the fuel pump relay, where it is indeed red-white. The FP relay uses the coil pulses as an indication the engine has spark and should require fuel, so it provided a nearby place to connect the tach. Easy job.

If you've connected the tach to the minus side of the coil, your car runs at high rpm well but the tach is reading only a small amount, your installation connections are probably fine; either you have a defective tach or one from a 6 cyl. car.

But no, I don't know exactly how the harness is built to get the red-white wire to the FP relay, coil and ignition controller on your 81. The coil primary is wired with coaxial (usually gray sheathing) cable in the later cars.
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Art Benstein near Baltimore