I just installed my snowcaps (got them from volvo dealer, price 14€ ~17$).
What a b**ch!
It's all very easy untill you get to the "slide the clips forward untill you hear a click" part.
I eventually managed to instaal them, here are a few tips:

- For what it's worth I put some fine (sewing machine) oil on the interacting parts.
- It's best to have someone help you: one pulls and the other presses down on top of the snow cap.
- I used a (fine) F-clamp to hook behind the sliding clip and pull it backward. Mind you I did not tighten the clamp (that would break the plastic) I just used it as a means of grip and leverage.
- You need a rediculous amount of force to click them into place.
- Watch out tha you pull the clip towards you (to the front) and downwards a bit (so you dont break off the little tab that provides a bit of grip on the clip).

Hope this is helpful to anyone.

Dietbrand Vandenberghe