"I dont know who manufacturers these windowscreens"
Sorry to read of your problem Juergen.
Supeficial quality on the cheap---sounds like the hallmark of the orient, specifically China, dont know how one would check this apart from asking the factory directly,although in their defence, I can report that in a recent conversation with Charles Morgan he did say to me that their company policy was to source locally and NOT the Far East. The problem today though is that its near impossible to know where any component is physcally made, even if your supplier is local!!
Hope you get it sorted soon.
Regards David of SW19
