My wife has the same issue. She is short and puts the seat so far forward she cant make the mirror work. I am 5'11 and the side mirror is fine for me. I hope you find that something is wrong. I hate to think that mercedes didn't think of this issue.
We wedge the mirror out for her.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660