Factory Tach problem

[BuickGS65]

I cannot believe that I can't solve this problem on my own. I suppose I could, by ripping apart my dash wire harness, but I'm not that crazy.

Anyway, I have factory console tachs in both my 65 Gran Sport and 65 Skylark. The tach from the GS is factory, the Skylark tach I bought recently. (There isn't much info on tachs in any of the Buick manuals) ....

So here's the deal. I cannot get the Skylark tach to work in the Skylark....
On my GS, The tach plugs into the console wiring harness through a brown slide-on clip. In the engine compartment there is a brown wire that is in the firewall wiring harness that goes to the negative side of the coil....tach works

On my Skylark, I plug in the tach to the brown console feed, but there is no (brown) wire that goes to the negative side of the coil in the engine bay.
What is the deal??????!!!!!

So then I tried to connect the tach using my own wire to the negative side of the coil. No luck...The tach would sometimes go haywire, to work at all, or give a little shock.....
Whats the problem?

BTW can anyone out there fix my tach if it is broken? I once heard that there is a fuse in the tach???

Thanks!

[70 gsconvt]

I've been having the same problem with my 70 tach. I'm told that it's most likely a bad ground. Since you're getting intermittent readings on the tach, that may be your problem too. Try running a wire to a good ground and see if it works.