Misting is usually because of the fact Minis leak. Moisture builds up in the carpets and gets on your windows.
If you strip your carpet out and put it out to dry/let it air for a bit it will probably improve the situation, you might want to check all the seals and replace where necessary as well.
Some rubber mats in winter or where your feet are more likely to be wet are a good idea, then the moisture just sits on the mat and doesn't soak into your carpets. Then just use an old towel to dry up the rubber mats when they get wet.
Dehumidifier bags are good as well. I have three; one in each door pocket and one on the parcel shelf. They only cost a few quid but work surpassingly well.
As AVV IT said, give your heater and ventilation system a good service as well. Duct tape all the joints in all the ducting to make it air tight and brush out your heater unit and matrix (matrix might even need replacing). The foam seals in the heater unit can break away causing air leaks. It's a good opportunity to give your cooling system a flush and fluid replacement as well.
The reason why Mini heaters/demisters don't seem to work well is because all your air is just leaking out all of the joins, then by the time it gets to the window vents there's not much left.