That should come off with a bit of elbow grease, it's gone that way due to car washes, I find most the Mercedes I work are damaged in the same way.
You need a chemical cleaner (unless its fucked). Try Werkstat Prime.
I had a friend with his 206, he used plain white toothpaste and a lot of elbow grease. worked really well.
Thats exactly how I brush my teeth also, thanks