I think you need to prise the mirror itself off first to gain access to some screws which secure the cover cap in place.
That. Press the glass mirror right in at the car side, stick a screw driver in behind it and prise it off slowly. It WILL come off although you'll feel like you're going to break it :S Once off there is 4 clips at the back that you just need to push out and the cover pops off. Takes 5 min once you know how