I don't know if you have A/C but a lot of interior odors are created by the evaporator. The evap builds up a lot of moisture and if it isn't properly drained then mold can build up and create foul odors. Newer Mercedes C-classes are prone to this, we have a special machine pretty much dedicated to the C-class
. You should be able to find a product to spray the evaporator and kill all the bacteria and mold on it and hopefully that will give you a better smell out of the vents. We also use ozium, it seems to be a good temporary solution. Good luck, hope this helps!