Door mirrors are a nightmare when you need to fit them to your B20!
The car body is so nice and pure without them....
If this helps, here from my 3rd series the picture of the door from the inside, and the fitted mirror (by the previous owner).
I think, but cannot confirm now, that the inside of the door is accessible.
However, the window might be in the way, and taking it out is not for the fainthearted.