controls to tachometer wiring question

I have a 1999 50 h.p with merc.commander 2000 controls, I bought a tachometer adapter to plug in to harness at controls,Purple wire is switched power and that is fine, gray is sending unit and Blk is ground,When I tested the power wire with voltmeter it test fine,except the ground wire is not grounded coming from plug in harness, I assume it should be grounded in the controls somewhere but must not be.any reason I cant just take that to a ground and then to my tachometer,I any wiring the whole boat and installing gauges,it never had any. Thanks for your help.

Any ground will do.I took the ground for my tach direct from the neg busbar under the dash.