well that's where the issue was on my last car. after thinking about it a bit i think the path is electronic and picks up the electrical pulses through the ignition points as they open and close to fire the spark plugs via the coil.
So it simply "counts" the number of times the points open and shut per minute - which is twice per each revolution of the motor. or you can try this route, You can get just the tath, you just need to remove the meter assemble and take the clear cover off, then there are 4 small screws
. If you want to start by checking those screws first to make sure they are tight before you replace the tach, that would not be a bad idea.
This web sight is a great link to finding and replacing parts with out the mark up of dealer prices and yet getting Nissan parts.