OK, here is what little bit I know about tach's.
There are two types that I know of, not counting the new ones connected to the computer modules. The one type has 3 wires connected to 12V, Gnd, and Point's side of the coil. The other type has 2 wires, one is connected to the ignition switch and the other to the coil's 12V terminal.
If were talking about 80's CJ's then the 6 cyl and 8 cyl tachs are the two wire type and are connected in series with the coil. Spliced into the power line from the ignition switch. The 4 cyl tach is the other type.
If you look at the wiring diagrams in your Haynes book you'll see the two different hook ups.
Now, how does the 6/8 cyl tach work? Here are my theories. It works like an AC ampmeter. When the points (or transistor in the module) close then power flows from the ignition switch through the tach, then through the coil and through the points to ground. When the points open power stops flowing through the tach, so every time the points open and close there is a pulse in the power line that the tach picks up. Convert the pulses to a voltage and measure it with a meter. I believe what they use inside the tach is a small transformer a couple of diodes and resistors and the meter movement. I havn't taken mine apart yet but what I could see down the light hole was the small transformer and some diodes.
My tach was made by Thomas G Faria Corp, Patent 3005155.
That Patent is to old (pre 76) to be online, at least I couldn't find it, but I did find a newer one by "Faria". US04059799.
(If you get a not available error on this site try it several times)
That's where I took the attached picture from.
My guess is that if you removed the two wires going back throught the fire wall you would have the stock tach.
Now how to connect it up to the computer or ignition module? Don't.
Connect it between the ignition switch and the coil ( ) terminal. I'm assuming the coil is still powered from the ignition switch.
Be careful connecting these tachs up, if you hook things up wrong you can burn things out. With the "Series" type tach power flows straight through. If this doesn't make sence to you don't hook it up.
This is my limited understanding of tach's. If someone knows anything more speak up before this guy blows up something.