They're fairly easy to replace, the trick is not to snap the tabs on the old one, or the mirror glass when removing.
Gently ease the mirror glass from the mirror housing, works best if you can get something behind either side, as close to the centre as possible, they're easy to break (this I've proved).
The caps are held in by 4 thin tabs which need removing, I used stereo removal keys to hold them back.
give the mirrors a good clean, fit the new cap by pressing in place, refit the mirror glass and voila!, repeat the other side.
Took about 30mins all in, most of that spent on the first one.