More than likely your instrument cluster does not have a good ground, which is a common problem on Mercedes cars. To remedy the situation, attach a separate ground wire (brown is preferable) to the back of the cluster (you will find a screw that holds one of the gauges in place to attach one end of the new ground wire). Locate the steering column support where the other brown ground wires are attached and ground the new wire in the same location. Make the new wire long enough to ease the removal of the cluster in the future if needed.