Green wire on the ignition coil.

Is the rest working as it should? Warning lights, illumination etc.?

In most cases it's just a mechanical problem. Open the tachometer unit and check the movement. You should be able to move the needle just with little pressure from your little finger's tip or even by blowing at it.

If it's stuck you'll just have to free it and make sure it moves again as it should. Fine mechanics oil or sewing machine oil is the key. Don't use something like WD-40. This is not an oil, it's mostly a cleaner