How do you get rid of the odor from the cars heater.

What is your question?

Since about a year after the car was new, a foul odor appears when the heater comes on. Tried changing Cabin Filters, spraying a lemon deodorizer into vents, dealer has did there fresh air treatment a # of times, I dusted the cabin filters with baking soda, I tried putting the heater on high-run the car with windows closed for an hour or two, but all has failed.