I have 2 tachos, one has a backighting circuit and 2 wires, the other has one wire. The other two on the one-wire seem to have 2 other terminals but those wires go into the unit itself with (2) 90* degree terminal ends.

Anyway, I want to try to install one of these. My 122's coil has only one terminal that goes to the distributor. This must be the negative. Do I need to break into the armored cable to get to the other necessary wire to make a tachometer work? I tried the one wire directly off the coil terminal with no results. I know I could get power for the backlit one from the back of the ignition from the pole that is on when the key is on. as for the others I have no clue. I tried searching this topic with few related results, if there are others detailing this modification I'd like to be linked to them. Thanks
Joel