1994 Dodge Intrepid Heater Core Replaced

The Heater Core was replaced about four days ago by a chain repair shop. We are experiencing the same problem as before. The interior surface of the windows are fogging up to the point it is severely hampering a clear view of the road. Is this a symptom of left over coolant residue that will correct itself as time goes by? Or, should I be taking the car back in and if so what should/can they do. Is there a solution I can use on the windows that will better repel this film of oily residue?

Thank you for any help you can give us.