[ref]mewneeb[/ref],
Very old thread revival so not sure how active the users will be but I replaced my passenger side one as it didn't work when I bought the car.
Upon take the mechanism out it was obvious why. The plastic housing had ruptured meaning there was nothing to tense the cable anymore.
I used a repair kit off ebay initially but the pulley that came with it didn't fit.
My suggestion is message the seller first, or do what I did and just message some people that are breaking Z4. That's what I did and I got the whole mechanism. Saved me having to run the cable again or anything.
Was relatively easy to do using some YouTube videos etc.