2015 Prius C has a mold smell that varies in intensity and seems to be worse on more humid days coming from the vents. Toyota says they are aware of this and have been for a whle but offers NO solutio

Do run the car with the fan on and the AC off to dry out the cooling coil which will reduce the problem, use the outside air setting, not recirculate though as you want fresh air. You may want to talk to Toyota directly and complain Maybe they will do something for you. I do suspect you live in a humid area which is not their fault. You could also use a moisture absorber in your car to keep the inside dry when you are not driving the car.

Could try spraying something in the outside vents by the wipers to kill the mold smell or at least mask it. Run your ac on outside air coming in .That's what the Car Talk guys say.

It seems that the water drainage pipe of Air conditioning system is clogged and water accumulates and then mold picks up. Or there is mold with in internal system. So the blower assembly and related components under the dash board needs to be opened and cleaned.

I would keep a box of baking soda in your vehicle to help absorb the moisture and smells. You could use an air freshener that’s has anti bacterial properties and you’ll use this by recirculating your air conditioning at full blast, then allowing the can to spray all of its contents into the vehicle with all windows closed and doors too. Allow this to work its way through the entire vehicle for about 25-30 minutes. Then park your vehicle for about a day with windows down in a closed garage. Change your in cabin filters before you do this! In cabin filters should contain charcoal because of your mold issues. I would repeat this every month or as needed. The filter shouldn’t cost more than $10. The air freshener I bought off of Amazon for $6 for a pack of three. I chose to get a smell that was floral. My friend sprays Tilex down where the air comes through and then allows the air to run...not the smartest idea, however it works for him. I recommend what I stated above.