If the smell is from the internal heating system and not from the exhaust system then this is likely to be due to organism (mould, bacteria, ...) growth in the air distribution ducts and heater. This is a well known problem. Proprietary products are available that are intended to cure this problem and adds are some standard household products will work in many cases.
I'd personally try a spray based on "Benzalkonium Chloride" (or one of the many name-variants. (Quaternary Ammonium Salts, Lauryl Dimethyl Benzyl, .... .... ....).
This is used in the majority of home disinfectants, and in many cleaners, moss & mould removers, carpet shampoos and more, at typically around 0.1% concentration. Using it in the same manner as mentioned below has a good chance of working.
HERE is a typical discussion with commercial products being suggested. They note
- You can do an anti bacterial treatment for your A/C by yourself. There are special antibacterial cleaners that you have to spray on the air filter once a month. You can also try cleaning your air conditioner at least once in a month to ensure that it produces clean and filtered air. Try products like Klima-Cleaner's. This product removes odors from the car and removes water condensation on the air conditioner system too. This can cost about $15 per bottle and can last you a long time. Other products you can try include GardX One Shot Deodoriser, and Kleen Air-Air Conditioning Cleaner & Purifier. Both of which cost about $45-$100.
[HERE(https://www.youtube.com/watch?v=CzYlp9dm7AI) is a Youtube advert for a "3 minute" fix. Efficacy and cost unknown to me.
Just found this - SE 2011 on the same subject .