Yes the correct part number is 72117334637
This is for the drivers side of an M3. apparently the M3s are brushed finish the 335/328 are polished finish
Aussiem3. you emphasize some good points. there are multiple things that can fail. If it wont come out, it is an entirely different problem. I tried to emphasize, perhaps adnausium, what my problem was, so readers could see if they were having the same issue. I have not checked the codes. I would be amazed if it knew the seat belt was not being grabbed. Though I would not put it past it.
The following link (about three posts down) has another video. this guy pulled the extender out. I didn't want to do that. The video is also in some foreign language.