I would only recommend this, if your vehicle can be in the open air, windows down, for at least a day.
One third bleach and two thirds water in a spray bottle. with the engine off and the windows open, spray the bleach/water combination into the vents of each ac outlet. let the mixture set for at least an hour, windows down. after an hour, hold your breath and start the engine. turn on the ac and let it run full blast for at least 15 minutes. the bleach will kill the mold and mildew, spores and the smell. safety is the name of the game here. plenty of ventilation and do not inhale the fumes.
If you are afraid to use the bleach, this will also work. Lysol spray. shake the can and spray into the left vent facing you from the drivers seat. ac off. spray the Lysol until the spray can be visibly seen coming out the vent to the far right. let this sit overnight. next morning, start the engine and turn on the ac full blast. your smell should be gone.