There've been a number of posts that discussed removing stuck spark plug connectors.
The pros on this site provided me with methods they use. Jeffsr(hands of Goliath) has mentioned before that you can twist and pull - I must be a wimp.
Then I discovered a special tool made by Hazet. The part number is 1849....not to be confused with 1849-1 or 1849-2 if you happen to have access to a Hazet catalog.
With Hazet Spark Plug Socket Pliers # 1849, spark plug connectors pop right off. I also
have the MB tool made by Lumen and it will "sometimes" remove a plug, but with alot of effort, even when using methods provided by the pros on this site.
They're not cheap, but then again, neither is an MB car. Special tools make a BIG difference. I suspect the folks at the PartsShop on this site can get them.