I just installed this setup, and it looks and works perfectly. The cover around the mirror is best snapped off by inserting a very thin bladed screw driver at the seam, where the two halfs meet, and carefully pry them apart.
The mirror wire most definately needs to be installed before the mount is attached. The two pins on the mirror wire are very hard to insert into the wire connector, but need to go in at least half way for the cover to go back on. Even then it will barely make it.
If you drill a 3/16" hole in the back of the mirror cover, where the two halfs meet, it is perfect size for the mirror wire to exit, and the cover be flush.
If you screw up the cover and need another, the part number is 1K0858548 (right) and 1K0858547 ( left ). About $20 at ECS. Best setup I've seen for a radar detector on our cars. The mount is very high quality, and IMO, worth the money.