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Just wanted to update regarding this:
I tried the desiccant method with a variety of different variety of cat litters as suggested by many sources only to discover efficiency was very low. 100 or 200g of removed moisture per kg of material seemed typical.
Some of the litter cooked after a few uses and rendered unusable.
With this I got some dehumidifier bags on eBay. Something like £10 for two of them, carrying 1kg of silica.
The hardest bit was to actually regenerate the silica.
The bags hold anything from 300 to 450ml of moisture when saturated at an ambient RH > 80%. A typical electric oven is often not ventilated enough to extract this amount of water and the air inside saturates. Result is that moisture removal is very, very slow.
Running on a hot plate or a ventilated oven at 120 Centigrade seems to have done the trick, but it takes a significant amount of time and energy for each regeneration.
So anyway, what I did was to get a normal microwave and stopping it from running on steroids. I chose a magnetron power of 250W (input power = 350W). This is continuous power, not the on/off cycle to average 20% duty cycle.
The bag is warmed up for 4 minutes on a normal 900W microwave (1300W Input) and once at temperature is placed on the low power microwave for 150 minutes. I normally take it out each 45-60 minutes and shake it a bit to mix the silica. By the end the bag has anything from 0 to 50ml of water left and the maximum temperature I measured was about 110C, so no risk of damaging the silica grains (I've been testing this for about 5 regenerations so far and the collected moisture hasn't changed).
The microwave is ventilated by default, and since the electromagnetic waves heat up the water, the bag heats from the inside, where most of the humidity is.
Each of my bags collects anything from 30 to 40ml of water per day, depending on the RH. This is not enough to remove the steaming on a wet day when using the car (and bringing wet clothes/shoes), but its good enough to keep the car dry if it is not being used.
The saying that such bags can last months is a myth. After 10 days the water extraction starts to gradually fall. By the 20th day the bag is saturated, unless once pours water over it, the silica grains are not going to absorb any more moisture from the air. I normally regenerate between 1 and 2 weeks. I'm in England where RH is typically above 70% and often at 90%.
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