Is your cable that goes in the tach square or more rounded 

The cable end is square, not rounded off. I believe the problem is not the cable at this point. While it is running I can put my ear to the tach and hear the inside parts spinning but the needle does not move.

I figured out why the tachometer stopped working. There are metal tubes in the gauge housing where the light bulbs slide into, and one of those had come loose when I was replacing the bulbs. They are definitely not supposed to be loose but the vibrations over the years must have broke it. That loose tube in the housing prevented the needle from moving while the cable was still working just as it should. I re-assembled the gauge with the tube in the right spot and it works again.