Tach testing

[reefrunner7]

How do I check if I have power to the tach my tach is sealed or so it seams, the only thing I can remove is the light bulb, any ideas? (my tach is stuck on 4k I have one loose wire yet at the motor unacounted for gray one coming from the backside of the wire harness,(that would be from the engine to the plug before it plugs in the the plug from the helm)