Are you using a six point socket or six point wrench. If you're using an open end, or a 12 point, I expect that a six point boxend would get it off unless you've attacked it with vise grips or channel locks already.
BTW. Make sure you have a new plug ready when you get that one out.
'01 C Class, Six Speed
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'88 300E 5 Speed
'81 300D Daughter's Car
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