The defective lamp problem is common on W210. As some have said...It's possibily the contact at the rear lamp housing. The contact with the bulb housing will fail due to arcing.

solution: Remove the bulb holder, and then GENTLY pry the metallic spring contact to increase the contact. If possible pry it out of the nearest detent and then use crazy glue to glue it back to the housing. Cheap solution no need to buy a new housing. Mine has held up 3 years and counting.