2002 ram 1500 2-wheel drive - replaced heater core over a year ago because it quit blowing heat alltogether. Now, hot air only comes out of heater vents and defroster when vehicle is moving. As soon as I stop it blows cold. ANYONE know the answer

 

Did you check the level of your coolant. I seem to remember, from past experience many years ago, that what you are describing is symptimatic of a low coolant level.