Personally I use it as provided here by the original author.
If you're looking for more damage maybe try the High level variant. I personally tend more to dirt than scratches or damage. So the Low version is enough for me.
It says "external". I noticed that the scratches are not visible from the outside. Maybe that is still to be implemented.
I too noticed an issue. It's with the K100. It has two windows. Scratches are only visible on the left one, no matter if I "buy" the scratches for the left or right window accessory slot.
But I don't know much about modding.