I found what I think is the easiest fix for the seat cable problems. Here is how I fixed the headrest:
I took off the rear of the seat and used a razor blade to make a cut all around the cable. I chose to make the cut as close as possible to the motor since I would be needing to pull out the metal cable afterwards.
I made a 2nd cut about 1/2" away from the first cut and unpeeled the cut area exposing a 1/2" space of metal cable covered in grease. I slid the 2 cables towards each other and held them with my hande while I tried the seat control. The headrest started moving up & down so it was fixed.
I pulled the metal cable out entirly and slid it through a piece of heat shrink tubing about 4" long and slid the other half of the heat shrink tubing over the other end of the plastic cable and slid the 2 plastic cables together again and used a lighter to tighten the heat shrink connector and it was done.
Now the cut in the plastic cable will stay together.
The fix was less than 5 mins, applying the heatshrink tubing was another 5 minutes, but be sure to use a paper towel with alcohol to wipe down the area where the cuts were made otherwise the grease will prevent tape or heatshrink from holding firmy. I also used a zip tie to align the cable in such a way that there was less pull on the repair when the motor is activated.
I am going to use this method to fix another cable that raises and lowers my seat. I will have to tilt the seat back for the repair but the beauty is that you don't need to disassemble the motors or use any tools other than a razor blade, a lighter, & heatshrink.
Fred in Houston

94" E34 540ia
86" E28 535i
83" E28 533i