Had this issue for a while.. I left my gym bag in the car overnight and woke up to a musty car. What worked for me was spraying some lysol/disinfectant spray through the AC intake in the engine bay. Its at the top right under the drivers side windshield wiper. I removed the cabin filter before and put the air on full blast and sprayed after switching between front, rear, bottom, etc vents. It will smell like the spray for a week or two but eventually it goes away. Guessing it was some kind of bacteria that the spray got rid of.