If you can I do think a dehumidifier is a help. The Carcoon will keep the air moving, more so with the vent open, but if it's cold and the relative humidity is say 80% you can get condensation forming on surfaces and even if the air is flowing, it won't stop condensation on say the inner wheel arches or behind the sills.
The other advanvage of a dehumidifier is that if the car is damp, say you got caught out or it's been washed, you can put the dehumidifier up to max and it dries the car out in a couple of hours. Before I had one I put up a thread about putting a car away damp in a carcoon, it was into a cold garage and inside the wheel arches it was still damp 24 hours later. With a dehumidifier running it's bone dry within hours.
Oh one quick tip of the month, everyone knows not to put the roof of the carcoon on till your exhaust cools down or it melts. Well came back from a spirited drive and went straight into the carcoon without stopping as the garage door had been left open on purpose. The floor of the carcoon where the tyres were melted a tiny bit, not through but just crinkled a tad. Worth stopping outside for a few minutes if you come back at pace :thumbsup: