Adding New Tachometer

[2ifbyc]

I have a 95 Force by Mercury 70hp outboard on a 17 foot sea pro. The boat currently has no gauges other than a fuel gauge. I would like to add a new aftermarket tachometer. I know how to connect it but I am having trouble locating and identifying the correct signal wire from the engine. Anyone know how I can determine which wire and where exactly to find it at the helm? Most of what I've read seems to be either a grey or a purple wire. Also, should the wire be there just waiting to be connected or will I have to cut an existing wire from somewhere? Any help is appreciated. Thanks guys

 

[haventaclue]

There should be, depending on the control you have, a plug or socket for a harness from the control to the gauges. Grey wire is the tach signal.