I recall when I changed the mirror glass on mine, that there is a screw behind the glass that can be tightened to make the mirror more stable. Carefully remove the plastic mirror seal, then remove the glass... If your seal is old, it will crack or break. Probably best to have a new one on hand before you do this. They are pretty cheap.
Good Luck!
Bob
bpossel
Memphis, TN.
1971 280SL
1997 E320