I have to agree with Jeff.
The motor is placed underneath the seat and controls both sides of the seat adjustment. The bar/link that goes from right to the left has to be damaged somehow (broken coggle left or right or broken link bar). You have to take out the seat to check what the exact problem is.

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)