It could be no flashing or weep ho;es over the window or you don’t have an ant-pooling fibro and the end of the rafters, it could be heaps of things I would check them all.
Try taking some titles of the roof to look in between the wall cavity to see if you have flashing over the window, it's not a easy thing to pin point<O:p</O:p
A dehumidifier only fixes the symptoms not the problem.
All I know there should not be moisture like you have inside the windows, if you can't find why I get someone in like a damp expert cause that crap ain't good for you. I hope this helps