I will take a pictures of where the sender attaches to and post it, unless someone else has one....otherwise the bat and grd are self explanitory. I just changed motors and went through the same thing with an 82 merc 115.
If you have an existing tach just mark the wires then remove them and hook up to a new tach.
There will be three wires. Gnd, +12V and signal. If you have illumination for other gauges then you will also have a wire for the backlight illumination.