Factory Tach Install?

I have a factory tach that didn't work well (response was way too slow) and picked up a reporduction one.

My car is a 68 Barracuda.

You have to remove the dash cluster to mount it. The ground is picked up by one of the mounting screws into the cluster housing. Then a wire needs to be attached to switched 12 volts and the other goes out through the firewall to the negative side of the coil (or to the tach output on an MSD). Lighting comes from the bulbs already in the dash.