The only practical way to sterilize is bleach (or extreme temperature), and I'm sure it means at least disassembling the dash. Vinegar is an easy way to kill many types of mold, but it won't disinfect. I imagine the evaporator comes after the cabin air filter, but you may want to find an aftermarket HEPA filter replacement anyway.
An N95 dust mask will reduce mold exposure, but you'd need a P100 respirator to stop it. They're pretty comfortable, and hilarious to drive wearing.
A problem with mold allergies is they include mold spores, which will be in the ducts, carpets, seats...since allergies don't differentiate between alive/active or dead/inactive spores, cleaning means physically removing or destroying the spores.
I'm told there is a Kia Technical Service Bulletin, and several different threads discussing this issue
. It happens with many cars, but is mentioned with Kia frequently.
Any water staying in a car long enough to grow mold is bad for several reasons. Maybe there's a way to add a drain hose to the evaporator somehow.
There are car air ionizers...won't kill the spores, but it'll help make them stick to things instead of float in the air. They also produce ozone, which helps with odors, but will aggravate respiratory allergies. An ozone generator running in the car may help oxidize mold with high ozone concentrations. There's a Canadian product called "Emzone OdorStop
" (I have it, haven't tried it), a sort of "air freshener bomb" that circulates alcohol/ethanol vapour in a closed car to kill microorganisms. Running the heater will dry out the ducts, and dry conditions discourage mold. If nothing works, try other dealerships for a second car, or bars for a hypoallergic wife.