Problem with Defog and Heat
 

I have a 1997 1.5 L dual cam protege. Just lately I have been having a lot of problems with the heating systems. When the heat is on and the car is idle, it revs on its own. Up and down. However when I turn it off, the car runs clean. Then yesterday, my caution defog light went on, but it still functions. Does anyone know what I should do. Please reply. Thanks. Kurt