I looked in the EPC (parts catalogue) for a '99 W140 rear door and can see every piece used in the door. There is nothing that looks like the part you've shown.

Is it possible it was left in the door during previous bodywork or work on the door? Does it have any MB logos or numbers on it at all? Perhaps it's just a rubber block of some kind that was used and left behind..

Are you sure it is from the door and wasn't either a)sitting in the trunk before you started the repair or b)part of the antenna replacement project??

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