You could try this, but I am not a tech or a pro, its juist my thoughts.

Liquid wrench or wd-40 let it sit on it for a while, then attack it again.

If all else fails, remember to oil the threads or use some anti seize on the threads next time.

Alon

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'92 300CE - Sold
2004 C240 - 744 - C7 Wheels - Android Radio
2002 C320 - 816 - Sport Wagon