The inside of the windscreen and other windows can get covered by a film of condensed plasticiser which leaches from the dashboard in very hot weather and to which air-borne dirt particles adhere. It is usually transparent and unnoticed until water vapour condenses on it, turning it opaque and so difficult to see through, especially when the low sun is directly ahead.
I have had some success with various solutions to try to shift it, ranging from vinegar,etc. as mentioned above and including methylated spirits, cellulose thinners & ethyl alcohol – all with the car doors opened, of course!
There has been no easy way to rid the screen of this tenacious layer – everything I have tried needed a lot of elbow grease to make an impression. The thing is, it is not oily or greasy, but I suspect is deposited plasticiser and so may need a very specialised chemical to shift it – such as the polish they used on the Apollo rocket nose cone ?
Good luck with your endeavour and more power to your elbow(s).
Edited By brian roberts 2 on 15/10/2021 18:55:59