I think they are sort of glued on there. Usually when auto body workers have to do work, involving rear quarter panels, they have to remove the window as well. The problem is, 9 times out of 10, they end up breaking the window, and have to replace it with a new one (which won't matter in your case ).

Again, don't quote me on it, but I know that it's not really held in with a mechanical device, or parts, like the front windows are.