AFAIK, the '87 300D (W124, OM603) does not have "fiber optic" lights like the W123 (and W126?) cars do. At least not if your '87 is like my '87.

What our cars do have is tiny individual light bulbs in all of the switches, controls, etc., all of which get their power from N40, the "Instrument Illumination Control Unit," located behind the instrument cluster. This acts as an "amplifier" so that the rheostat in the instrument cluster does not burn out due to the load of all of the little bulbs. If N40 fails (mine did) or if the fuse (#9) for its power fails, all of the lamps except for the instrument cluster will not work.

You may have pulled a connector loose when you removed the cluster to install the new temp display. N40 is bolted to a bracket alongside N2/4, the "Warning Module." N40 is the right-hand one of that pair.








Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970