190E 2,3 16v 1988- heater controls illumination

[nicolas20]

Lighting to heater/vent control knobs and surround not working. All bulbs replaced and fuse checked including small pcb fuse in the red holder in rear of instrument cluster pcb. Do you have a copy of the wiring diag? Understand the centre bulbs are backlit via a plastic tube but I am unable to trace the feed to the twin wires (grey/blue & brown) which are connected in series to the heater controls and bulbs in the control surround.
Regards,