Hi Adrian,
Welcome!
I'll give it a start. Mostly the 2 tiny cylindric rubbers (sprockets), on the stick (part 3), are dried out and loose their grip when you turn the stick. Maybe you can pull/press the headrest by hand? I'm not shure if part 105.44.58.94.10. must be unscrewed when you want to take out the headrest. The spare stick I have has the 9,5 mm sprockets (105.44.58.863.13). It comes from my former 2000 GTV. The stick can also be removed, when you'r lucky you can read the partnumber as discribed on Tavola 172 (item 3).

Good luck!