Avoid the vise grips, you'll ovalize the nut, turning it into a locknut.

Use a sharp chisel and a small hammer.
Use quick sharp raps with the small hammer to let the chisel get a bite on the sides of the nut.
Eventually the nut will begin to turn.

Destroys the nut, but you'll be able to remove the part.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022