Meaning there is a poor ground connection at the cluster. By "connecting" the back of the instrument cluster with the body, you gain a good ground. Just remove the cluster and remove one of several screws on the back holding the instruments to the cluster. Take a heavy gage wire 10g and attach a ring terninal to the wire. Then screw this wire to the back of the cluster. Take the other end of the wire and attatch it to the metal dash support visiable when the cluster is removed.

This will improve the ground connection.

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