My mothers 530 does this as well, at least it moves down. really bothers me because I have a 300 foot long driveway that has several turns in it and I like to use the mirror to see where the shrubs are. My father took out a post light once, but that was in a rental. You cannot see a thing once the mirror moves down. I do not think it is supposed to move all over though that would really serve no purpose. I would end up in a tree!

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Currently
1991 Mazda Miata 174000
1991 Subaru 189,000 Winter beater

got rid of:
1984 VW Vanagon 195,000
1976 350SE Euro 185,000 KM
1973 280SE 4.5 (boy do I regret that)
1980 280SE European W126 4 speed manual (miss it)
1989 BMW 735iL 119000 (good riddance)