Sometimes folks put the wrong bulb in and it melts the plastic around the contacts - the contacts then stop making contact. Same thing happend on my '97 W210. When you open it up you can bend the contacts so they hit the contacts on the mounting. It's pretty easy - do a search in Tech Help you'll find other threads on this topic.

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'97 E320 Black Opal
'86 300E Smoke Silver / Burgundy (retired at 198K)
'63 Austin Healey 3000 Mk II (BJ7) (reluctantly retired)