warm moist air touching cold dry surface = condensation
I'd guess all the water emmiting from the concrete made everything moist and as your car warmed up the cluster warmed quicker than the surface of the plastic. Sounds to be a really moist place to store a car, not a really good thing. I would leave the windows shut to help stop the quick temperature changes and hopefully leave the inside of the car more stable.
There are (dont remember what their called) bags that you can put in all 4 floorboards in a small bucket that attract and hold moisture from the inside of the car. No clue where to find something like this, maybe a boat facility?
Good Luck!
95 E34 530I V2.37
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Those who make peaceful revolution impossible will make violent revolution inevitable.
John F. Kennedy