The tach signal is on the grey wire. You will need switched +12VDC (purple wire) and ground (black wire). Depending on what kind and year of control you have will determine where you can tap into these wires.
at tha gauge or at tha motor?... what Chris says is at gauge....
at motor you have two reg/rectifiers under tha coils.. both have a grey lead.. grey in tha harness can plug into either of these....
