I know this would never happen, but let's just say someone left their sunroof open on a hot, sunny, Oregon day in spring.
And let's just say they spent the night with family and when they woke in the morning, this was pouring rain.
And let's just say everyone in the family listens to Car Talk on NPR and knows that mildew is the worst enemy in a situation like that.
Here is what I would recommend:
1. Close sunroof and towel dry inside of car
2. Gather some quarters and use the vacuum at a car wash (with their permission) to see if that can vacuum up any moisture
3. Fill your tank with petrol
4. Start engine
5. Leave one rear window cracked open
6. Let engine run with heater going for five hours
Not that I know this for sure, since this is all hypothetical, but when you return to the car after five hours, everything will feel as dry as the Atacama Desert, including the carpeting and seats.
Will this prevent it from suffering mildew?
I'll let you know.