After checking what DADS50 says and trying to get contact without modification, if cluster can won't contact gauge ground plate and you prove that is the whole problem with this isue,
then:
If your ground plate looks like the pic DADS50 just posted, you can solder a wire to the solder blob and run the wire to ground. I have seen some gauges that have a round slug between the studs that is not supposed to touch anything. If yours is not like that and the center hole is just to clear the solder blob, then you can solder a wire. Be careful not to get things too hot and lose the existing solder connection. You probably should use a non flame solder gun and probably don't need to add solder. Don't let the solder flow to a bigger blob or it won't clear the hole.
After soldering wire and assembling gauge into cluster can, you may want to epoxy the wire to outside of cluster can as a strain relief.
The proper way to solder a wire to something is to "tin" it first. This just means twist the stripped strands, get wire hot and apply a thin coat of solder to wire. Solder should soak thru all strands.Then solder wire to the ground plate solder blob. Heat the wire until it sinks into the blob. That ground plate is a "heat sink", so may be difficult. Do the soldering with gauge outside of cluster.
