Stoney

... several products designed for the purpose which are both harmless to the car and are designed to leave an permanent active coating to prevent it from coming back.

Diluting bleach doesn't reduce its chemical activity in corroding car parts (both in solution and as a gas as it evaporates), but just reduces the rate of reaction by reducing concentration and total quantity. You want a fungicide that is intert chemically, bound to some sort of polymer which can adhere and last a long time.

ChrisS -2008 A6-4.2

the source has been completely removed. I'm not an expert (didn't even sleep at the Holicay Inn Express last night) but, I experienced a similar problem last year.

1) I would be hard pressed to believe under any circumstances that the metal or plastic surfaces in the HVAC system are capable of absorbing and then giving off a mold or mildew smell.

2) I could be convinced that you have a point of moisture ingress or a blocked condensate drain that has allowed the re-growth of same.

3) I had a windshield replaced (Seattle/Rain) and shortly after experienced a dreadful smell coming from the vents.

The resolution was found in sealing a spot between the windshield and HVAC ducting that allowed water to reach the pollen filter.
Once sealed and pollen filter replaced the smell was gone for good.

Stoney

... hence my interest in a lasting fungicide. Unlikely that a single treatment with Lysol (plenty strong enough to kill what it can contact) will reach every crack and pore.

Finman

I wasn't actually suggesting to do it...I would use something "designed" for cleaning air systems in cars...but dilute bleach sprayed in will kill the mildew...quickly and will evaporate quickly...it doesn't hang around for hours, days, weeks, etc corroding things. I certainly would never bring any bleach into my car...but from the engine side, perhaps...spraying bleach inside a car, one would have to be an idiot.

Finman

especially in relatively humid climates that are cool...hence one does not use their A/C much...but use it intermittently and it may have condensation sit in one place or another...
Fungus/mold is in the air at all times...it just wants to find a nice moist spot to grow. Lots of A/C systems are like this...it is very difficult to remove all condensation just from gravity...and if it's humid, it has no real reason to evaporate.

Stoney

Of course it would go into the car's interior when you clean the vents... it has to pass in to reach the problem.

I don't see why you suggested a product you would never use for that purpose, especially when there are specifically-designed products that have been tested for this.