Josep : You are absolutely correct; the screw is where they get the ground connection. So, unless you are the owner of some "invisible wires", you'd need some kind of a clip or "pin prick" device (screw point or something like that) to make the ground; either of which would probably scratch the paint.
I guess that's one of the purposes of these Forums; to "kick things around" and bring up things which may have been forgotten or not thought of in a Reply.
Personally, I have no problem in adding an Accessory item to my cars, as long as they are Genuine Ford, and can be documented as being available for the year car in question.
Bob