It is not uncommon for the sun visor hinges to break.
Car Problem Reports
Mercedes-Benz SLK320 Hinges For Sun Visor May Break
Mercedes-Benz SLK320 Problem
Average Mileage: 72,500 mi (65,000 mi - 80,000 mi)
Visitor, 2002 Mercedes-Benz SLK320, 80,000 mi
Driver side sun visor broken detent
Visitor, 2004 Mercedes-Benz SLK320, 65,000 mi
Cheap plastic drivers side sun visor won't stay up. The Sunvisor itself is cracked and torn and is generally not up to the level I would expect from MB. This was made during the Chrysler Daimler era. Looks like the was an American influences part. Apparently I will have toder new because I did not see any on line.