Since nobody else is helping, I'll chime in with an educated guess (based on how it's done on other vehicles): You probably remove the triangular trim piece on the inside to reveal mirror bolts. It may be necessary to remove the door panel before it's possible to remove the triangle thingy.
If you can find a site selling mirrors with features that weren't available (like an in-glass turn signal arrow), they're likely to have instructions available.
Can somebody tell me how to remove front mask on golf GT mkV. I cant find any tutorial