It goes right here in the harness:
Or in a little more detail:
What you do is to seperate the plug and insert the tach wires. THis is because the CJ tach is a SERIES tach meaning it goes in series with the device.
Unlike aftermarket tachs.
And yes, since it acts as a wire, it will kill the engine right away it you try to hook it up the conventional way.
In your case it will be:
Batt-------Ign Switch-----Tach------GM HEI (+12V)
Or the one side of the tach will go from your ignition switch and the otherside will go to whatever terminal supplies power to the distributor. DON'T hook it to the "Tach" terminal.