That is what happened to mine! I scanned the instructions, and made a drawing showing the simple fix to the internal 'v' spring that will repair your old one, and attached them below:
Pic 1 mirror assembly, showing spring loaded pin/clamp that secures mirror to roof
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PIC 2, INSTRUCTION SHEET (note that when installing or removing mirror it helps to have a multiply folded terry towel between windshield and mirror in case mirror slips during reinstallation
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PIC 3, INSTRUCTION SHEET
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PIC 4, drawing of inside of mirror on C230: The 2 'V' springs can fatigue and allow the ball joint to flop around in the socket. With the mirror out of the car and on a flat surface, the front face of the mirror can be CAREFULLY pried appart, exposing all the internals. Bending the 'V' springs to excert more force on the joint fixes the problem (thanks to repair instructions provided by member MbenzNL!)
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John
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E