I would suggest just ordering the mirror caps from Audi. The trick is removing the mirror to access the screws holding the caps in place.
[video=youtube;XqRLGV9aFS4]https://www.youtube.com/watch?v=XqRLGV9aFS4[/video]
Found these instructions from another site.
- Tilt glass so you can get your fingers behind it.
- Pull the glass clips out. (I managed to get one side loose, then needed to tilt the glass in the other direction in order to get my fingers behind the other side).
- Once the glass is off you will see x4 screws which hold the black plastic trim section on. Remove these.
- Remove the mirror adjustment unit (centre screw).
- Ease / unclip the black plastic trim section towards you. (there are x4 clips you unlock in order to do this, x2 top and x2 bottom).
- Once you are inside the mirror you will see x2 further plastic clips at the top of the unit. Unclip these and slide the Cap away from you without lifting.