When I turn the headlights on, the lamp indicator comes on. However, all the bulbs are working. Other than randomly replacing bulbs, how do I troubleshoot?

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Probably the license plate bulb.

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1993 e300
1995 e320
1994 e320
2006 s500 4matic
2004 Jeep wj overland
2001 Ducati 748
2004 Honda shadow aero

Yes, I was just coming back to edit my post but you beat me to it. The right license plate bulb is out....my oversight. However, I have replaced the bulb and reseated the connections and it still doesn't work. So I guess there must be some upstream continuity issue. There is no voltage at the leads.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Yes that sensing relay is known to introduce failures in the bulb circuits. It has a series connection to each and every exterior bulb, and if a path opens up inside the relay you have no voltage at that bulb.

Exception, the turn signal bulbs. It instead takes a sample of the voltage post-flasher and looks for the rapid 'bulb out' flash pattern.

Fortunately every 124 and 2nd gen 126 used this exact same relay so they are easy to find.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Investigated a bit further. Could get the bulb to go on and off by moving around the wiring cluster where it comes into the trunk lid. So I'm guessing that's where the problem is. BUT even when the bulb is on, the indicator is on....so maybe I need to look into the sensing relay.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife