Alright... let's get our terminology correct...
First, there is a drive cable... that metal inner cable that motor uses to
move the seat...
Then there is outer cover sheathing...
By cutting the outer cover sheathing shorter, you are making the inner drive
cable 'longer' that will insert deaper into the motor drive.
The problem with 'whirring' motor is that the inner drive cable is not
catching the motor drive. So by shortening the outer sheathing, you are
making it easier to keep the inner drive cable in contact with the motor.
With the motor installed and clamped in place, it is unlikely that the inner
drive cable will jump out as it is pretty much held in by tension. It has
been a long time since I done this... I did it right after I bought the car
back in '93 and never had to touch the cable to date.
As I said before, it is pretty common occurance on W126. I think what
happened is the grease somehow affected the outer sheathing... causing it to
swell a bit at the connection or causing it to slip out just a bit that the
inner drive cable became too 'short' even though the actual length has not