Mildew in Stored Car

We live in SC and we store our Honda in our garage (which doesn’t have A/C)for the summer when we’re away. Each summer there’s been a little mildew in the car, so this year my husband left the car windows open with the car in the garage. This made it much worse. Every leather surface was covered with mildew - none on the vinyl. You could not tell the seats were black leather. THey looked grey/green. Spent several hours cleaning it off using mild bleach solution first, then vinegar solution a week later since it still smelled. I have scrubbed around all the seams with toothbrush. Still smells a bit. Any suggestions? Should I take it to be detailed?

But what I really need to know is how to stop this from happening in the future. The summers here are hot and humid and so is our garage. Would it be better off left out in the driveway? Thanks.

My suggestion would be to make sure your garage is well ventilated. Maybe a small exhaust fan or something like that. The problem is that when the garage cools at night, the humidity condenses into dew and when the garage heats up during the day, that dew evaporates and guarantees that the relative humidity in the garage is always 100%.
They make small solar powered ventilators to prevent this problem in sailboat cabins.