IB;

I will venture a guess; that tach is probably 12V. A lot of the earlier 6V tachs have separate sending units. Can you find a model number? A model number or some ID could give you something to research. If it is a "current sensing" instrument the "Dist Pri" wire usually connected to the hot side of the ballast resistor. There are a few gauge restoration shops on the net that might be able to steer you in the right direction. BTW, cool truck.

SimS