pop the hood with the engine up to operating temps, theres 2 hoses off the alt bracket to the fire wall, carefully touch them both, if they are both hot, thats good, if one is warm the other cold your heater core has air in it.
take those 2 hoses off the alt bracket, put a garden hose to one hose and flush it till a steady stream of water comes out the other hose, then put it back on.
if both hose's are hot like they should be, the blend door actuators could both be bad. http://www.grandprixforums.net/showt...door+actuators