[canamfan]

Considering HVAC setting as mentioned, also ensure your heat exchangers/hoses in & around engine for heat are absolutely tight & in good condition, clear from any allowable intrusions of engine oil and fumes. However slight & tiny any cracks, openings or misaligned/loose connections there might be, will make it possible for burnt oil/fumes & even smoke to make there way into the cabin as that same residue collecting on windshield. You should (when needed in winter) be able to keep warm air/defrost setting @ desired setting "without" any fumes/oil residue entering the cabin if all components are in top shape. Check it out to be safe.