Use any soapy substance on the inside of the windscreen i.e. shampoo or washing up liquid that will stop it from fogging up too . I`ve had to use it in emergencies before when the dehumidifier fans in a vehicles stop working and it`s lashing rain outside and lots of passengers inside causing the entire windscreen to fog up .
Here`s another professional drivers tip . If your windscreen wiper stops working cut a potato in half and rub the cut part on the outside of the windscreen this causes the rain to run off the screen . The only problem is that in our beautiful land of the potato you`ll never find one when your driving along and this problem strikes

Note to self must keep potato and soapy liquid in cab .