If I remember correctly, the tach on the 66 charger only has one terminal on the back, and that is the + side of the meter. The minus side of the meter is the case of the meter, which should be hooked to ground. If your tach has two wires, it could either have a + and - for the meter, or it could have + for the meter, and another wire for the light inside the tach, with the case of the meter being ground. You can usually take these tachs apart without damaging them if you are careful, and you can look and see if the two wires go to the meter movement or if one goes to a light. The suggestion with the meter was a good one in order to find out which wire is positive if both + and - terminals of the meter are available. I sometimes pop a tach with +12V for just a very short period of time to see which way the meter moves. Do not connect 12V to the meter for more than about 1 second or you could fry the meter.

Later model tachs (67-74) usually have two wires going into them (D for - side of distributor, and +12). The case is ground on the later model tachs.