Coupe rear window question

Well on my 66 dart I have tried to roll down the back seat windows on the car and the feel very stiff. If that makes any sense.
It just feels like somethings going to break when trying to roll down the windows.
Is there anything I can do to roll down the rear windows easier.
Or is there something I can maybe grease up to help the windows roll down easier.
I know its a weird question but I just dont want to break something.
Thanks!