There is a single motor on the rear seat mechanism. It has cables that run to a plastic gear on each side of the seat to the slide mechanisms. You may have a cable that's come loose, a stripped gear, broken gear, etc. Remove the lower seat cushion (2 10mm bolts at the bottom) and observe the mechanism as you operate the switch. If you see the cables rotating, you know the gears are the fault.

I haven't attempted to repair mine, getting to the gears is easy enough though. I got the mechanism in the upright position and disabled the motor to keep it from getting jammed again.

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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]