If its just fallen off, best bet is to buy 2 new runner poppers, there like 79p from ford, as time goes on, the poppers wear and pop out all too easy, slie the poppers into the runner, align the ball joints on the end of the mech and with the new ones, you may need a pair of grips to pursudae them to pop back on.

Make sure theres plenty of grease on the runners too, as most sierras iv'e fitted door glasses on can be quite stiff to move