Old cars often have deteriorating interiors(not my Honda) and I got to thinking why. Obviously the sun is a big culprit, the sun will fade dashes, make leather crack, plastic bubble, etc. Why does this happen? Because all the moisture has been evaporated off the surface, it has essentially been dehydrated. So, how do we keep things hydrated? With hydration of course, which then begs the question, how much hydration? Too much and your glass gets foggy on the inside and you start getting nasty musky/moldy smells, too little and the interior starts to degrade. I mean if you put a small dixie cup in a car, and fill it 1/4 way, and leave the car parked in the sun, some of that water will evaporate to steam increasing the humidity in the cabin. Now humidity in the air does have the benefit of limiting radiative heating from the sun, and it also has the benefit of saturating interior parts limiting sun based dehydration. But there's the problem of smell and mustiness that comes from swamp weather.