power window issue

ok heres the trouble, my sister has a 99 buick century, her rear power window seems to be totally dead. the switch will not move the window, and if it is pushed up she says it will fall back down with some driving. any idea what this could be? i figure all these are just about the same in most cars.

could it be the window motor? or would it likely be something more on the mechanical side--- since the window almost seems loose up and down.

thanks in advance