"Condensation on mirrors.
To prevent this from happening simply wipe down the mirror with a damp cloth with a spot of washing up liquid on it and let dry naturally. The mirror should remain clear for up to 6 weeks. Not only does this work with household mirrors it will also greatly assist with internal rear view mirrors in cars and also when applied to the inside of a car windscreen has a very good effect. Taxi drivers have been doing this for years especially in winter months."
my driving instructor used to use this method over 20 years ago