Tacho signal wire identification

[bobfather]

I need to pick up RPM signal pulse inside the dashboard so I'm trying to find which wire delivers the coil pulse to the tachometer. If I understand this properly there are three wires, +ve, -ve & signal. Is there a way to identify the signal wire? '99 Chim if that makes any difference.

[Steve_D]

At the tacho
+12v will be green
-12v will be black
Signal will be white/black. (this wire may start as white/blue at the coil)

Steve

[bobfather]

Thanks Steve