Tach works when I hose down my engine bay

[BlackTomC]

Just to clarify, the ground running from the tach on my setup is currently running to that common ground. So you're saying that it shouldn't be running to it?

[BlackTomC]

So I ran all new grounds:

Battery to rad support
Firewall to head support
Rad Support to frame
Alternator bracket to head
Head to Frame

Also made sure my headlight harness grounds were up to snuff and secured.

So here's some more info, I pulled my tach out and traced down all the wires. I have three (black/brown/pink), the harness running from the distributor to the tach is fine, I replaced that a few weeks ago, so signal to the tach is great, the pink wire running to the fuse box was all janky so I cleaned it all up and plugged it back in, then the ground had completely fallen off this little 6 prong dealio up under the dash so I repaired that as well.

Now I seem to be getting a better response from the tach but it still isn't operating as it should. And when I push my horn button, then let up off my horn, my tach goes down, then back up... Any ideas?

And my dang interior dome lights don't work.. can't figure that one out either..