The windows, and especially the windscreen, in my S-reg MX5 and 51-reg MR2 started getting terrible condensation now that the weather's turned colder/wetter.
Having Googled around, I thought I'd give the cat litter/tights method a go.
You need a bag of Bob Martin's Felight and a pack of 5 nylon tights (they are official “denier” tights, I'm surprised the AGW people haven't shut them down yet). Total material cost about £9. You cut the legs off the tights and pull one inside the other to make double thickness stockings, tip 450g of the cat litter crystals into each double thickness-stocking, tie up the ends and put one on the rear shelf and one on the dashboard.
With the newer MR2 it has worked a treat, there was barely any condensation at all this morning, even though it was raining all day yesterday. The windscreen in the slightly older MX5 is still misted up, but nowhere near as it has been. The downside is, some of the finer powder makes it way through the double-stocking and there's white powder sprinkled across the dashboard.
So now we know!
I'll just have to find some use for the last pair of tights (wrong size for Mrs W)…