If it used to work, the wiring is probably not suspect. One failure mode I know of is that there is a calibration pot, and after 36 years of not being adjusted, the connection between wiper and windings corrodes. The fix is to exercise the pot and then recalibrate. In my case, the car is modified anyway, so I just replaced it with an Autometer Monster tach -- fits right in with minimal grinding of the dash material. You could put the original back in and no one would know anything had been modified.

How did you fix your speedo? The problems I've had is with the worm gear slipping forward an causing the spinning magnet thingy to contact the pickup. Easy enough to put it back where it should be, but I've never managed to make it stay there. ???