I just bought a 2003 santa fe and the owner said he had a "heater core" issue, but put some stuff in it and fixed it?? I smell antifreeze in the vents, and the windshield fogs up like crazy. I can't even keep it clean just driving a few miles from home to the store and back. I don't notice any drips and the floor is dry. How hard is it to just change the heater core altogether?