You can temporarily use a piece of wood (2 x 4) with a screw or nail in it to hold up the windows until the correct regulator can be sourced. That's what holds up both of my W210 rear windows; the previous owner had me do it for him when the regulators both went out a couple years ago, since nobody ever rode in the back seat. I'll be replacing them soon...

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280