I don't know if it can be saved- my experience with gluing plastic has not been great. However, I do know you can remove the mirror itself from the base- assuming it's the same as the later year S3s- and hopefully find a replacement. If you fold down the small triangular rubber flap on the near end in the photo (the one down at the base), you can use a metric allen wrench (I don't remember what size) to loosen the mirror from the base.
Remove the inner door panel, look up in there and you will see how it is attached. I think the only way you could save this would be in a box in the garage. Worthwhile as you may need the painted shell or movement mechanism later on.