They're on the side of the transmission. You'll see a bar coming from the shifter to the crank on the side of the transmission (driver's side). There's a plastic bushing at each end. There are tools made for installing the bushings, or you can get creative with a bolt, a couple washers, and a socket. I did the latter, but it depends on how much room you have to work with in there. The 126 had plenty of room to get in there. Not sure about the 123.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

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

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]