I also recently changed the mirror covers on mine. They came with instructions from Fiat.
Best ignore them! I tried to find the rotary clip that is supposed to hold the mirror to the mounting, but gave up 20mins of struggle. Remembering this thread, I tried the "pull and pray" method. After a heart stopping crack, it popped off cleanly, without any damage. No sign of the rotary clip system behind the mirror! Also, the instructions didn't mention that 4 out of the 5 clips that hold the cover on are only accessable by removing the 4 cross-head screws behind the mirror!
Easy job, as long as you don't follow the supplied instructions!