In the wiring loom it's before the ballast resistor although I don't think it would matter. It works by sensing the current flow through the wire and is not physically connected. I doubt that 6 volts is significantly safer than 12 (after, before). Mine's never actually worked unless it's plugged in right at the coil, not sure why but I'm working on it. It worked a bit when I had the dwell set too long on my electronic ignition and burned out the coil.... Next thing is to try 2 loops (double the strength of the signal to the sensor).

But that could also double the counts of the signal, maybe?

Nah just the strength, the coil of wire is like an electro magnet. Because it's pulsing it works the same as moving a magnet past the sensor in the tacho, two coils means the magnet's twice as strong.

Get's a bit tricky unless you know about electromagnetics. Hope that makes sense.

Yeah when you explain it like that. Made electromagnets back at school.

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Inline to the + side of the ballast resistor. That's how Datsun wired it from the factory. I hear you can also do it from the - side of the coil and perhaps even from the resistor side.

If you wire the + side, you've already got that wire in your cabin (it's on the IGN switch), so it will save having to run dedicated wires from the NEG side of coil back into the cabin.

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So between the resistor and the coil?

Between the IGN switch and the resistor.

So before the resistor then?

What's this guy on about?
http://www.shoeys.com/pics/coupdsh.jpg

The Datsun factory wiring is before the resistor. But you may be able to do it otherwise.

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He's about putting a Coupe dash (early round gauge cluster) in a sedan that has wiring for a rectangular gauge cluster.

Apparently you can wire the tachometer multiple ways and it will work OK.

If you use the factory wiring, or splice into the POS side if the IGN circuit, you don't have to run wires from the engine compartment into the passenger compartment.

But do whatever seems easier, reports are good.

A few fuses is a good idea just incase something goes wrong. I often use very light aligator leads (wire with clips on the end) to test things. If something goes wrong the test leads burn, not my expensive tach/ part.