A few months ago all my tell-tales and gauges went bonkers on me. Erratic, nonsensical behaviour. I knew that many things weren't wrong with the subsystems, so after some time of chasing everything down it turned out that there was a common ground wire from the dash-pod to the panel under the drivers window.
That one wire was mounted into the bottom of the case which houses all the connection points, and that wire's connection to the case was corroded in, around and under the bolt which secured it. Cleaned/sanded mounting area, replaced bolt/nut, and viola! All peachy again.
I found it by measuring & discovering quite a bit of resistance from some of the locations on the back of the gauge panel to ground, then to test the finding, running a ground from a known location to a known location on the gauge-pod and everything started behaving.