Do a search on here as I put up a thread a while back about the rechargeable de humidifiers that my dad uses in his caravan, I now keep one on the back seat of each car, been in there about 4-6 weeks and doing a good job on moisture, still not saturated yet either.
They contain crystals, but are of course re useable, charging also dries them out for re use.
About £20 a pair, even come with a attachment hook for each so can hang on coat hanger in car.
If in storage if you can leave a few windows partially open will help.
tabetha