They are really easy to break. Basically there are two small tabs that slide into a metal slots mounted under them at the base of the glass. The metal slots are bent in a way that let the tabs slide in easily, but bite onto them when you try to pull them out. In the pic above, you can see the tabs have been broken off which is what usually happens when anyone tries to remove these trim pieces.
I have made a tool that I slide down under the trim and "push" the tabs out of the metal slots, but to make such a tool, you almost HAVE to see one removed and see what exactly is going on under there and where the tabs are located.
Those trim pieces are shorter on the Capri models than they are on the Mustangs, so Mustang pieces won't work... and since I own 4 Capris, I snag every set I find at junkyards and have gotten pretty good at getting them off without breaking them... thanks to my special tool.