Repaired high mount brake lamp with LED still have failed bulb idiot light
 

W124 300E Just curious if the current flowing through the LED is too little and still triggering the idiot light. The entire top half of the high mount lamp essentially disentegrated as I took it apart to change the bulb. Even the plug cracked in half leaving the leads dangling. I mickey moused an LED light in there and it works fine, but the idiot light makes me nuts. No lights are out, checked them all including the plate frame bulbs.

Sounds like there a resistance mis-match. There are commercial LED bulb replacements that are ohm match rated.

The third brake light bulb in my '97 w210 is not in the sensor circuit, so maybe yours isn't either. Mine was out and never triggered the message. You could "mickey mouse" in a correct bulb to see if it stops the message. The w210 has a problem with the brake light contacts on the tail lights that will sometimes give a message and still be working, at least when ever you are looking at them. Don't know if your model has the same problem or not.