Well since the windows are blown out, you don't have to worry about making them...right? Just frames. So I'd draw yourself a template, and make some frames flat over the picture, out of pastillage or gumpaste, ahead of time so they'll dry and be sturdy.
Mmmm, scratch that, nevermind. The whole bus will be solid cake right? So I would probably paint the window part black and either paint on the little shards of broken glass, or make some out of isomalt.
painting the window black was my first idea and if someone else came to that conclution i think i will go with it