Ground problems are hard to find sometimes.
I've had pretty good luck using a booster cable and bridging it across
normally connected points to see if the problem goes away. Even after one
finds what bridge fixes it, it is often still a head scratcher where the
actual missing connection is.

To the OP, you might want to try this: use a booster cable's negative
connection only - don't use the positive side at all. Tie one side to the
battery's negative and bring the other side to any point that should be
connected to ground. (engine, frame, dash, etc -- make sure you know what
you are doing, because connecting it to the wrong point can cause damage).
Most exposed metal pieces are tied to ground. "walk' this cable across
connections to see if your problem goes away.
Once the the booster cable makes the problem go away, you'll have to figure
out where the missing ground connection is - often a loose screw or bolt
holding down a wire to the frame is the problem. My bet is that a connection
from ground connection to some major component in your dash  is missing or
bad.
This will take some detective work.

Remco