The two connections are ignition-switched +12V and coil. The tach grounds via its connection to the metal instrument panel housing.
I believe the tach makes use of the same printed circuit on the back of the instrument panel. The traces that would otherwise go to the gas gauge get hooked to the tach instead. One of the traces is a switched +12V so it stays the same. The other trace was the gas gauge sending unit. The different wiring harnesses take care of re-routing that trace so it gets connected to the - side of the coil instead of the gas gauge sending unit. I suppose you could always cut that wire (tan) on a non-gauge cluster. Re-route the instrument panel end of the wire to the coil. And re-route the gas sender end of that wire to the new gas gauge location.