So, when i turn on the air, or vent, there comes after 5 minutes a really bad smell. I don't know how to describe it, but its BADDD. The smell only occurs when the air or vents are open. It's been like that ever since i got my car last year, but it's bothering me now b/c its summer and i need the air. Does anyone know what the problem is??? How do i fix it?

Thanks guys!