dashboard dryer - dehumidifier

You'd be amazed at what a difference cleaning the windscreen properly will make. Go get a glass cleaning product, and thoroughly clean the inside of your windscreen. You can buy special anti mist products at places like Auto5, but clean the windscreen first. I usually buy a couple of bottles of Rain X when I'm in the U.K.
http://www.halfords.com/motoring/engine-oils-fluids/screenwash-de-icer/r...

I assume there is an equivalent product available here.

If you want to go with the dehumidifier bag route, you can buy them at brico.

Cat litter in a sock works very well. Just pour cat litter in a sock and stick the sock under the seat.