IIRC those bolt heads are a goofy size like 12.8mm, they are tight and your socket must be in good shape and square on the head... Try to remove adjacent bolts and remove the stipped ones with a pipe wrench. Pipe wrenches exert force in the jaws in proportion to the force placed on the handle...

Lots of luck finding replacement bolts at AutoZone in the morning.... I believe that the manual calls for all of them to be replaced once removed...

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Over 21 years I owned several:
w108
w110
w111
w115
w116
w123
w124
w126
w212