I saw your post and set out to investigate. I looked into a 1988 560SEL. Remove the four 8mm screws that hold the rear seat motor in place. Snip the zip lock to get it out of the way. With motor free of car frame, you can pull the cables out. They do not pull out as easy on the geared track end. PIA in my opinion.

Look carefully at the cable ends that came out of the motor. They should be squared off. If all looks good, carefully pull the greased cable out of it's sheath. If the other end of the cable is squared off it is not severed. And is still good. It is obvious that the length of the greased cable be as long as the sheath it came out of. If not, you still have part of a cable inside.
Getting it out is another matter.

You will have to unbolt the geared end of the assembly and hope with a couple of flicks of the wrist, the cable section will come out. Then all you need do is put another cable in. I can help you with that if need be.
Inspect the gears for wear. You already said the motor works. Do the switches on both sides engage the motor? If not you may have a bad switch. I can help you there also.

Good luck

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