PP ( stargazer upgrade ) question

Sometime at the end of last year the RPM portion of my PP stopped showing. Not a huge deal but i was wanting to get it fixed. I got under the dash and looked and the black wire is just hanging down loose. I looked on the PP site and it said to attach it to the ground on the old tach. Well i dont have a tach so is this a common ground or does it HAVE to be grounded to the other side of the engine rpm sender. I wont be able to get it in the water for another few weeks to test but i figured someone on here could answer.

Common ground should be fine. Ground on a tach just goes to ground. They probably use that location as they want you to connect the RPM signal to the tach signal too so the wiring is close.