As mentioned before in this thread, this is not the only way, or necessarily the best way, but I had plenty of time and curiousity about how the seats work in general, so I kept digging deeper.
DISCLAIMER: I am not a professional mechanic, and have really only been studying the E39 for a few months. Use all this at your own risk.
PROBLEM & POSSIBLE SOLUTIONS:
My original problem was not seat back twist, but seat bottom tilt when using height adjust. In other words, one side of the height adjust was working and one was not, resulting in the seat leaning you towards the center console. All the general tips on seat cable repair apply, but accessing the motor/cable mounting screws on the sport seats appears to be more difficult than the comfort seats or other BMW models as referenced in many of the other online DIYs. (Several referenced here:
If you are reading this early on in your search for seat twist solutions, consider all the options:
1. Live with it. (When I first bought my car, the only position where the seat was level was all the way down, which seemed pretty comfortable anyhow.)
2. Get someone else to fix it (dealer,indy, friend, etc.)
3. Shorten cable sheath by cutting out mid section and rejoining with a piece of tubing and two small hoseclamps (seems to work for most, probably the easiest and quickest, method "B" from above)
4. Shorten cable sheath at motor end using heat technique without removing from motor (possible risk of heating something you didn't intend to? method "C" above) also mentioned here:
5. Remove cable end at remote mechanism and repair with heat technicque (may be possible for seat recline, but seat height cable connection seems inaccessable with out full seat cushion removal?)
6. Remove cable end at motor and repair with heat technique (method "A" above). This appears to be easier on other seat styles or other BMW models. See Photo 3 here: http://www.nmia.com/~dgnrg/page_11.htm Several of the other techniques are mentioned here also.
The problem with E39 sport seats is the bracket design and generally tight working space around the motors. Mostly this is due to fixed obstructions, but in the case of height adjust failure you may also have the issue of the seat being adjusted all the way down, putting additional seat mechanisms in close proximity. One pair of brackets attaches both the motor for seat height adjustment and the motor for seat recline to the seat frame. To remove one of the cables from a motor, you must either:
a) entirely dismount the motor (2 screws at each end) and slide it out, or
b) entirely remove the bracket on one side of both motors (4 screws, 2 per motor).
So this bring some additional considerations....
6A. Dismount entire motor assembly from seat, enough to pull away from seat slightly to access all 4 screws at one end of the motor pair, and remove the cable you are after.
6B. Bend obstructions to access enough screws (either one side or one motor) Also mentioned here:
6C. Cut/Drill to remove obstructions or create access holes (just a brief thought, didn't really take it very far)
7. Supposedly some people have also fixed this by welding extensions on to the actual shaft rather than shortening the sheath, but I'm no welder, and you still have all the same access issues.
In my insanity, I kept chasing 6A, so if you read this far, and didn't want to do #3, this is where the procedure actually starts:
(Keep in mind my issue was a height adjust failure on the right side of the drivers seat, but this could probably be adapted for seat back twist, passenger seat, etc.)
A) Remove headrest, unmount seat, seatbelt, and should belt height adjustment cable. (plenty of other threads on this)
B) Adjust the seat for best access to the repair: 1) get as much height from front tilt to give better access to motors, 2) extend thigh support enough to access torx screw under center of it, 3) adjust malfunctioning height until even, 4), adjust seat angle back slightly for easier access to the screw under the leather near the hinge point that secures the rear of plastic switch housing, and 5) don't adjust seat angle so far back that it's difficult to get it out of the car.
C) Disconnect battery (if car brain can't see seatbelt status and pretensioner, it will set error code - see other threads for details)
D) Unplug seat wiring harness (pull/pry lever sideway, connect should pop out)
E) Remove seat from car (better with two people if possible), place on workbench laying on its back
F) Remove switch panel (sorry, not much detail, but check other threads on switch repair for this)
G) Remove connector cover (one nut, one torx screw, plastic hook around bar) and tape back out of the way (wiring harness is still connected) You may need a flex shaft, u-joint, combination of 1/4" drive socket extensions to get to the torx.
H) Remove torx from bottom of thigh support, slide all the way out and fold back onto seat cushion
I) Partially remove leather from front left (opposite on passenger side maybe?) seat corner. It's not as scary as you might think; just compress the cushion and keep pulling the leather towards the barb until you have enough slack to unhook the leather from the frame. You will need to do the first 4 - 6 barbs or so, starting at the edge of the thigh support opening and working your way out.
J) Open/Unhook the upholstery rings (2 or 3 as needed) along front edge of main seat cushion.
K) Remove two motor bracket screws from top of seat frame
L) Remove two motor bracket screws from front of seat frame
M) Partially slide motor assembly out enough to access bracket screws and remove the problem cable end
N) Repair cable using heat & trim method (documented elsewhere, but this was my variation)
N1) Remove shaftO) Reinstall cable in to motor (some twisting may be need to align square shaft inside motor), and re attach bracket to motors
N2) Carefully pluck out padding / seal at cable end
N3) Heat metal sleeve (maybe ten seconds or so with a heat gun)
N4) Pull sleeve off with pliers
N5) Clean melted plastic from metal sleeve
N6) Push metal the 4 barbs back from inside sleeve to ease re-instalation
N7) Trim about desired amount off the cable with utility knife (I trimmed 3/8" because shaft appeared long enough at the motor end, but by giving it a quarter turn, it dropped back into the sheath about 3/8" when it re-engaged the mechanism at the far end of the cable.)
N8) Insert cable in metal sleeve
N9) Reset barbs into cable (I used a spring loaded centering punch while resting the cable in the end of a scrap of 2x4, but there's probably a better way?)
N10) Heat again / push on slightly with pliers (in theory helping it all stick together better?)
N11) Let cool and re-install padded end seal
N12) Lube shaft if desired and insert
P) Install motor assembly into seat
Q) Test your repair. I did this by temporarily connecting the seat and main seat control switch box, and resting it upside down on the door sill. (Protect seat and door frame as needed)
R) If good, remove battery again, disconnect seat, return to workbench to reassemble remainder of seat
No, this is not the quickest method. Can't really say how long because I spent a lot of time figuring out how to do it, and I also removed parts that were un-related because I was doing a lot of cleaning where the previous owner had spilled a sticky drink or two. Maybe a couple of hours? Is it the best method? Hard to say. Why did I bother documenting it? Well, I've really enjoyed all the help and information her as I've gotten to know my car better, and I wanted to actually contribute a little. Maybe not the easiest cable repair method, but this would also be useful for someone who needs to replace their seat adjust motor, or swap a good one from the passenger side to replace a failed one on the driver side. Anyhow, hopefully it will help someone.