The coil plug connectors are Resistors and they are Infamous for mis-fire failures on Waste Spark Ignition systems . They cause low ignition coil voltage at the plugs, so the plug foul after a while..New plugs will do the same in time..........
Any experienced Benz Tech will change these when they change the plugs b/c they are aware of that common fail condition , so it is standard procedure to change them at a regular plug change.
The ones on the open plugs do not suffer from this condition, only the ones under the coils b/c they are engine heat trapped and do not have the snap connection.

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A Dalton