If I'm not wrong there is nothing like mouse_pos.z. Mouse coordinates are in x and y. However even with mouse_pos.y there would be problems because my.z is in world coordinates and mouse_pos.y in screen coordinates. So you need to add some sort of conversion formula to make it move,which can be done with vec_for_screen() function.
Hope it helps
I like good 'views' because they have no 'strings' attached..