I'm going to add my experiences here rather than start a new thread. My car came from Oklahoma and I can only assume that the previous owner ran the car with the A/C on recirculate non-stop because the mildew smell was awful. I've tried to clean it before, but when I fired up my A/C a week ago, it smelled worse than ever. This is after I treated it with Nextzett Klima-Clean and sprayed two 1-ounce bottles of Frigi-Fresh through the intake. I decided to do a more thorough job this time.
Like before, I inserted a tube for the Klima-Clean as far up the drain tube as it would go and plugged it up with my finger. I emptied about two-thirds of a can up there. Here is the interesting thing: I had an endoscope through the main opening where I removed the blower. I didn't see a single sign of the foam. The drain is on the front of the evaporator, so spraying through the drain tube evidently only coats the front. It does not penetrate through to the rear.
This is what I saw:
For this reason, I would say that it's absolutely imperative that you spray both sides of the evaporator. It also allows you to clean your blower motor. I cleaned mine the last time I did this so it looked pretty good, but this time I oiled both motor bushings with Tri-Flow. It does make the shaft spin a little more freely. I sprayed the back side of the evaporator twice and then after about 20 minutes, I also pumped some distilled water in there to wash out what was left of the Klima-Clean. Last time my A/C smelled of Klima-Clean for nearly a month.
If you're thinking about going though the front vent, don't bother. I checked that out as well. The vent pulls off pretty easily and you get this view:
The vent closes soon after you turn the car off which I think is one reason mildew builds up in there. You have about three minutes to work before it closes. I put my endoscope down the circled area and found this:
I'm about 95% sure that you're seeing the heater core. It's too clean to be the evaporator. In short, don't bother trying to go through the vent because nothing will get through the heater core. Spraying anything down there is a total waste of time. It won't reach the evaporator and will only gum up your vent motors.
Tomorrow I'm going to buy a cleaner "bomb" and set it off while the system recirculates. That should kill anything in the vents themselves. If this doesn't rid my car of mildew smell, nothing will.