The adjustment pins are broken, Motor replacement is the only fix I know of. ( My 97 C280 has broken pins )

Does this car have auto adjust mirror ? ( if you have 1,2,3 and green button below the seat adjuster switches you have auto adjust. ) The auto adjust mirror motor has a 6 or 7 wires, regular 3 or 4.

Motors are the same left to right and across a few platforms, at least the 97 SL , C and E will interchange if auto adjust is considered. Electrically the auto / manual adj motor setup is different and won't interchange.