Could it be the cabin filter needs to be changed?
I have a 2012 Sienna that has started to have a mildew smell. I found the drain hose under the car and attempted to see if it was blocked. I used a piece of 12 gauge electrical wire and fished it up the hose. I was only able to get it in about 7-8 inches before it wouldn't go anymore. Should the wire just have entered the evap box unrestricted? The wire felt like it was blocked by some form of rubberThe AC must moldy smell is common in many cars.
I've heard people and dealers use this product. It works very well:
After you clean it, to minimize it from coming back, the best thing to do every time you are done using the AC, turn the AC OFF, and full blast non-circulate hot air for 1-2 min before turning off car. This will dry the evaporator unit to prevent mold build up.
You should also check if your AC drain is plugged. After you use your AC over an hour, you should see water dripping on the floor, which shows that your AC drain is working.