[Brett]

To wire in an aftermarket tach, you'll need the wiring diagram/color coding for your merc
in order to connect the proper terminals on the tach to the correct wires on the outboard.
Not difficult, just have to pay attention to the details.
I've had good luck with Teleflex gauges over the years.

 

[DuckNut]

A straight connected tach will give you far more reliable readings.