2000 Dodge Caravan problem.Please help.?

Anyone know what would cause our van to have a terrible odor when the heat or air conditioner is on? Defrost is even worse. It has a musty smell so bad you cannot stand it. We have to spray Oust outside on the grill part by the windshield to get any relief. So we smell Oust rather than musty. Husband took the cowl apart and checked it. It seems fine. What are we missing? Anyone have this problem? This is not an anti-freeze smell or anything leaking in the heater core or inside the van. Just a musty mold smell.