On the 126, there's a very small gap below the shock mount that's just wide enough to get a pair of vice grips in there. Not sure if the 123 is the same or not. If not, you'll have to gain access to the shaft somewhere, risk boogering up the threads on the shaft, or use a nut splitter or a grinder to cut the nut off.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)