It freezes from excess moisture in air.
That could come from door seals, floor mats (never understood how they don't supply rubber mats in a country where it always rains), sunroof, etc. More seen in cars that are driven only short distance.
Try heating car up well on long drive and turn heat up..... then open the windows (on a dry day obviously). Try using dehumidifiers etc.... or cat litter if you're cheap.