Touch the glass with your finger in all four corners. If you detect any free motion then the most likely problem is in the mirror follower jack screws. These can wear out if the mirror is adjusted to the stops frequently, or if someone has forced the glass out of the position provided by the adjustment motor. This is a sturdy mechanism, but it can be broken by any type of excessive force.. Good Luck..
1987 BMW 325
[This message has been edited by jeffsr (edited 04-03-2000).]