Had what sounds the same a few days ago. Passenger side glass jiggling at motorway speeds and looks like it could fall off any second. As far as I could tell the two options are:
1) Buy a replacement glass unit off Ebay. These are aftermarket and not identical to the BMW ones but seem to look fine when installed. Just be sure not to get the blue tinted ones as it won't match your other side. These seem to cost around £10.
2) Carefully remove the glass and re-glue in place with something strong like Araldite. Be careful as the adhesive pad behind the glass is actually the heating element itself so this cannot be damaged. I would suggest remove the entire unit (glass and backing plate) and then use something flat to carefully pry the glass away and loosen the small amount of remaining glue. Then clean up the glass surface and heating pad surface, apply glue and clamp together until dry. Reinstall into car.
I went for option two, and seems to be holding up fine for now!