Paul, I posted a lengthy description of the repair about six months ago.
I've also sent out scanned pages of the seat removal process to several list members.
There is a single motor under the front edge of the seat responsible for raising and lowering the seat cushion using two scissor-jacks located on both sides.
When one cable fails, only one jack works and the seat rises unevenly.
The jack gearboxes are located at the back of the seat, and two cables are used to transmit power from the motor to the gearboxes.
The drive cable consists of what looks like a tightly-coiled piano wire inside a protective outer jacket.
For some silly reason, BMW's cables fail due to the inner drive wire shortening inside the fixed outside jacket.
The solution is either shorten the outer jacket (my recommendation), or insert a short length of coat hanger into the end of the motor and reinsert the failed cable (untested, but I've been assured this works).
It's possible to fix the seat in the car, but I prefer to remove it by simply unscrewing the six big T-55 Torx bolts holding the seat rails to the floor and disconnecting the electrical plugs.
Once the seat is removed, simply loosen the failing cable from the motor (two small bolts and a retaining clip), remove the cable and the inner drive wire, carefully work the end fitting loose (hardest part), and strip back about 2 mm of cable using a hacksaw.
Attach the end fitting, reinsert the drive wire, and attach everything to the motor.
Replacing the cable completely is time-consuming since it involves removing a good deal of parts at the rear of the seat.
Furthermore, you'll soon have the same failure as the new cable shortens and disconnects itself from the drive motor.
The shortening procedure is the preferred repair method.
If the dealer understands the problem and will fix it for only $60.00, I'd strongly recommend letting them do it.
Fling, Mark F.