In your climate and without use it would not be a surprise at all if the ducts were full of mold. This could seriously affect your health so take caution. I don't know if it is easily accessible or not, but if you can get to the inlet for the air, you could try misting a disinfectant into the duct system to see if that helps. Otherwise, you may just try turning the heater on full hot and let is sit and run with the windows down for several minutes (preferrably without you in the car) to see if that cleans them out.

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Keep everything as simple as possible-but no simpler--Albert Einstein