No big deal, Tom. Of course probably all of these headlights nowadays have plastic lenses. I should have known it better anyway...
...or asked a second time.
So in the end its my own fault and confusion.
But these damn plastic lenses are so crystal clear and with all those "glass-like" marks and signs on it and it felt really hard and strong (knocked on it before), so that I was tempted to use the "other" side of the sponge to get rid of the remaining glue of the electrical tape.
Thanks for the advices for the car polish. I´ll give that a first try, ´cause that´s what I have at home.