series resistor to instrument lighting of the 1989 onwards digital instrument cluster, 9220.127.116.11
it is on D21/22 of the schematic.
This series resistor reduces the maximum voltage of the alternator to protect the bulbs in the instrument cluster. It is located way down near the brake pedal.
The dimmer of the instrument cluster background lights is a wire wound Rheostat of about 6 Ohms. It connects in series to another wire wound resistor. This resistor is about 3 Ohms.
12 volts from the instrument lights goes to the Rheostat, through the series resistor, and from there onwards the signal connects to the instrument cluster. This is called the 58d signal on the cluster. There is another one, called 58 which is permanent 12v when the lights are switched on. So the 58d only influences the dimming but will not work when the 58 signal is missing.