The right window is not openning

Hello,
i have a 3 doors opel tigra 98 model, the problem is when I close the back door (the trunk) the right window should open for an inch or so automatically, but it is not. I have checked the switch that should trigger the window to open, it is giving a signal, so it's working fine; now I suspect that there is a problem with the window itself.
please I need your professional help to tell me what the problem could be.
thank you