You don't say what model/year, but on our '85 760 a couple years ago the
splines stripped out (where Peter mentions). Replacement of the stripped
parts was a hassle, so I cemented the pieces with blue Loctite and tapped
the wiper on with a socket and a small hammer. So far, so good. Now I'm very
careful to make sure the wipers aren't stuck down in the winter before using
the wipers.