Yes it fixable. van Swaay is about correct. I had the same problem but it was on the forward/aft adjustment. Removed the seat, disconnected the by sliding out the circlip securing the cable specific to up/down motor. replace the steel cable (maybe cable sheared on one end inside the motor). Problem solved. Requires elbow greese too...

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1987 300SDL 314K mi, counting; #22 Head
1999 E300Turbo 405K original miles
1997 E320 180K mi (Son's)
2002 ML320 (Son's)
1998 S320 140K mi (Son's) sold
1985 300SD 311K mi
1984 300D 324K mi (killed by a hit-n-run turd)
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