2002 Dodge Intrepid heater

Hi, my question is if my car is just idleing my heater does'nt work, but works just fine if the car is moving. If I stop at a red light it starts blowing cool air but as soon as the car starts moving the air is hot again. My car does not appear to be overheating in anyway. And also if its really cold outside my dash lights dim and get brighter what could that be. Thanks Alot Patricia