There are two `types` of tach. Usually the older types are run by a cable that spins like the old speedo`s. More modern types are driven by a small electric pulse. To get this pulse, sometimes a magnet is put on a crank pully and this spins past a fixed coil that sends a pulse every time it passes. Sometimes a single small wire is taken to the alternator. I don't know but at a guess this will be how yours is run. A little bit of knowledge about alternators. First inside the alternator is 3 coils, each of these coils give off a AC current, This AC current is fed through a rectifier, that converts it to DC `Direct Current`, and through a voltage regulator, where that other wire goes to, so as to control the voltage and tell the coils when the battery is full. Right, back to the Tach. Basically a single wire is taken from only one of the inner AC coils, any one will do, and that will send a pulse every time it goes round a turn to the Tacho, that is basically a pulse counter, but the dial is printed off as revolutions'. At the back of your Tacho, I guess there will be two wires going into it. One will be for the internal light, the other the pulse wire. There might be a earth [ground], but usually this is done through the case to the metal frame.