Sounds like a dimmer is limiting the fixture to 6 V. It should go to 12 V at max adjustment. I don't know the W210. In my W123, the dimmer is just a rheostat in the cluster and a shot of Contact Cleaner often gets more current from it. Many LED's don't light at all until a certain voltage, and then you get almost max output. So, they don't work well with a dimming function. Doesn't bother me because too much dash illumination is usually never a problem in a W123.

Ditto, on "flip LED around and try again". All I have used are polarity sensitive. Doubt one could have damaged anything in your car. But, for those putting them in the dash, be very careful and measure resistance with it installed in the bulb holder. My M-B holders have unique fingers. Some LED's leads cause a short across the bulb holder, which melted a PCB trace in one of my clusters (repaired).

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans