Okay, I'm blind, (50+ years), the gray wire is there. The one thing I do know is wiring, checked all connections at the power head, all clean and tight. Ohmed the wire out from power head to tach and all is well, all connections at the tach are clean and tight also. I have not looked into the control box, but doesn't the gray wire travel all the way through? I tried working the trim switch while the meter was connected (ohms) but didn't get any flucuations there.
I know the power trim is a heavy electical load under power, but with a fully charged battery and working alternator I wouldn't think it would effect the tach in anyway. (never has) I'm going to try running a new wire (temporary) from the recifier straight to the tachometer, and a clean ground striaght back, I'll let you know how it goes. This should eliminate any interference. If the problem is still there, will have to be in the tach itself or the stator in some way. As you know the stator is new.