Get some antibacterial spray from the supermarket, own brand is fine, about a quid.
Start car, put aircon into outside circulation, coldest setting. Close all doors and windows.
Spray into outside vents at bottom of windscreen where the pollen filter is, spreading it all over the vent so it gets into the trunking. Do about twenty squirts and leave it for a minute. You should see it being drawn in. Check the car remains on outside circulation. I use about 100 mls but if the smell is stubborn do it again the next day.
You should smell the delightful fragrance in the car.
Did mine the other day, I do it every few months.