The 928S4 has a Rheostat (variable wire resistor) that is located under the dash at the left side.

To avoid too much heat dissipation of the rheostat there is another (fixed) resistor in the 12v supply line.

It has a bi-metallic switch that opens when there is too much heat generated. If the switch fails, obviously there is nu cluster illumination.

This is where the resistor is located. A little hard to find sometimes :)

close up:

regards  Theo