I have an '84 B120 [1200 ute] and there are no wires in the dash loom for the tacho.
As previously advised, you need to run a pair of wires from the tacho. to the points side of the ballast resistor.
One wire connects to the resistor, the other to the wire that was previously on the ballast resistor.
You are simply extending the ignition wire back into the cabin, through the tacho. then back to the ballast resistor.
As a security measure, you could put a hidden toggle switch in series with these wires to give you a KILL SWITCH
There should be short wire on the rear of the tacho. with a female bullet connector on it.
That plugs into the main instruments to pick up 12V, just like the clock did.