I used the earthing point in the boot which is just inside on the left behind the light.
Getting a feed from the battery was difficult for me, the cupra tdi has no room in the engine bay to get access to any gromets. In the end I went up the passenger side wing, down the door crack and in through a gromet there. The cable can only be seen if you are looking for it with the door open so was fine for me. I'm sure there are other places but I found it to be the easiest and most knuckle friendly