I think your cluster is not grounded. You should run an extra wire anyway, as this is a problem in the original design. Find a screw on the pc board to cluster housing that is a ground trace. You can easily do that just by tracing the copper foil up to one side of a lamp. Put about a 1 ft wire on there so you can get it in/ out of the dash, and hook to a screw/ nut hidden, such as in the column support yoke behind the cluster.