welcome! the dealer can run some cleaner through it, or you might find something at an auto parts store. i believe some have put baking soda on top of the cabin filter. all the best.
I basically did what is outlined in the YouTube vid below -- only difference, I used a product I bought from NAPA -- and it was pricey -- and I sprayed product through the dash vents on the inside of the car. Used the car for a day or so, and then did the reverse (sprayed through the vents on outside of car). Cavet, this was in my old Corolla, not in my Prius. Worked well. Here's the vid -- there are a few others on YT also:
What's your location? Are you running AC all the time, or? AC use causes a lot of humidity in the cabin air to be precipitated as water on the cold AC grill inside the ventilation system. Some of that water drains out through a tube, to the ground, but not all. If you run AC frequently, and run it right up to the moment you shut down, you tend to overload the car's ability to dissipate this water. It sits in there, gets funky. A couple of tactics to reduce moisture in the system: 1. Leave the AC off, when it's not sweltering hot, just crack the windows, or roll them right down, depending on your speed, conditions. A side benefit: it's good for mpg. 2. When running AC, shut it off a few blocks before your destination, to give the water more time to evaporate. This can cause a bit of "humidity dump" in the cabin, depends on timing.
make sure your cabin filter is clean and check for debris while you have the filter out. make sure the drain tube from the evaporator housing is dripping under the car when the a/c is running. if it ain't the condensation is sitting in the evaporator case and that will stink. get some frigifresh, ozium, etc. spray and spray it into the cowl vent with the blower on full just like in the video.
Yea, well except that the heater core is NOT where the mold usually grows. It usually grows in the AC evaporator and it's drain pan. The fix is to be sure the pan is draining properly and then pumping some disinfectant in through the air ducts and vent hose. The bad news is: once it starts, it is hard to eliminate completely. My year old C is starting to stink; totally expected in Florida. I'm gonna let my dealer handle it when it gets bad enough.
