I have the exact same problem with the drivers side folding mirror on our '03 - it won't stay in the fully "unfolded" position because there's a lot of play in its swivel mechanism. I checked the torx screws under the plastic cover on the bottom of the mirror, and they're all tight. Any other ideas? Thanks.

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2003 X5 4.4i
1992 Porsche 968
2007 328i
2014 Subaru Forester XT