I put my NC in my mother in laws garden for the winter and put a cover on it. Rather stupidly left it since October. When I opened it up water dripping off the inside of the soft top, on the seats and belts and showing as mould. Is this normal? Should I have left windows open or not bothered with the cover?
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Unused expect this, even covered up.
I’d probably just have an half cover for the soft top and regularly air the car and check for damp.
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Or just drive it through the winter (though not on ice/snow) and enjoy it. Wash underneath regularly when there is salt on the roads.
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If you shut it up in October the dew point of the trapped air would be 10-20 degrees higher than the sub-zero temperatures we experienced a few weeks ago.
The inside of your roof will have acted like a dehumidifier. Best to lock it up with ice cold, super dry air on a freezing cold clear December day - not warm moist October air.
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This isn’t strange. I’ve had my MX5 in the open air, standing still for a few months. It was very moist inside, drops to the roof etcetera. I was still a soft top n00b back then 
But I got it clean and fresh again and learned from it. Now I have a Cabrio Shield that only covers the soft top. It locks behind your doors and trunk lid so anyone can’t just get it off when the doors are locked. Knowing that, I always open my windows slightly when the cover goes on the soft top. This does seem to help against moist.
Agreed. Ventilation is the key!
When we stopped using our old Cavalier (with 9 months MOT and still hoping to sell it) it stood outside for a bitter, damp, icy winter, but both windows were open just an eighth of an inch.
I was amazed to see its insides remained absolutely dry and condensation free, even though my two year old daily-drive Astra standing next to it was often suffering badly and needing a couple of minutes to clear the windows.
I resisted the temptation to leave the Astra windows slightly open for fear of someone taking the opportunity of a decent grip and forcing their way into the car - no alarm.
Alas, the frameless MX5 windows need to be shut with a good seal to stop some of the rain running off the roof being driven into the cabin (and don’t forget the soft-top drains taking water through the inside of the car). Thus, if left slightly open a secondary cover is essential, exactly as Nippan suggests.
Buy a few pounds of rice, put it in a cloth bag (pillow case is ideal) and leave it in the car. The rice will absorb any moisture in the car. From time to time place the rice bag in the oven on a low heat for an hour to dry the rice out. This will work, is very cheap and you can have a nice rice pudding in the spring.
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I will add to this. I bought a bunch of plastic trays from Home Bargains and thens everal kilos of cat litter. I put a plastic tray in each footwell, 1 on each seat, 2 on the dash and 2 on the parcel shelf at the back.
Completely removes all moisture in the car, costs a fiver and it lasted me all winter.
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I never had the problem this year. Didn’t use a car cover and opened the doors once a week or so.