I believe I know the answer but will ask.

In wiring up a shift light, one of the shift light wires (green) is suppose to connect to negative side of the coil. Can the factory tach wire (e.g. the single wire coming from the engine compartment to the factory tach) be used for this connection? Or is it best to run this green shift light wire directly to the coil?

The two other shift light wires are a ground and an ignition source and are not in question.