Well... my first car, a VK Commodore had the same kind of setup. The glass lens was held on by metal/brass kind of clips. I got them all off, cleaned in and out, then tried to put them back on.
The clips were so tight, that when I got to the third one, it snapped on hard and cracked the entire lens. From top to bottom.
Got a better one from the wreckers in the end. So if you try it, bear in mind that you run the risk of making them worse.
If the crack goes all the way through the glass, there is nothing you can do to repair it, as far as I know. If it's just surface marks, you might have to use a cut and polish or some sandpaper then polish.