The "exterior light failure indicator lamp" on my '87 300DT is under the tach/clock, in between the windshield washer reservoir light and the ABS light. According to the owner's manual, if the indicator glows, it indicates a lamp failure. For headlights, etc., the indicator will come on when you try to switch the failed lamp on. For brake and turn signal lamps, the indicator will come on when you try to use that function and will stay on until the ignition is switched off. This suggests that there is an electronic circuit somewhere doing all this monitoring. Failure of the circuit might make the indicator lamp come on all the time. I'll do some research in the schematics, try to find this circuit, if it exists.

Jeremy

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"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