[ddfour1]

I don't think there is a relay. if you can get a meter on the power leads for the tach I would start there. Or try running a jumper from a good ground source to the tach ground and see if that fixes it. You can do the same on the +12v side if the ground checks out.

Good suggestions. Despite all of the ground wires looking free of corrosion and like they were making good contact inside the spade, I decided to cut the wires back and put a fresh spade connector on.  There are 4 ground wires that all converge into the single spade that attaches to the tach illumination ground post.  So far after doing that, (1) the panel lights no longer short out the tach, and (2) the tach stopped bouncing.  Maybe there was a internal corrosion or a bad contact inside the spade that was causing the issue.  Hard to believe since everything looked clean.

Dave