Someones nicked them because they're very easy to get off.. 5 seconds per side if you know what your doing! which is why they went missing, someone who knew what they were doing needed some!!
new ones are dead easy to fit, they just push into place, by gently pushing the centre of the glass, and spreading the load, until they snap into place, if you feel uncomfortable doing this.. take them to a garage, probably cost you a drink!
Tom