if both heater hoses are hot when up to full running temp the core is ok, if one is hot or cold or both are just warm, you got air in the core.

if theres no air trapped in the core its a blend door actuator. http://www.grandprixforums.net/threa...or-Replacement

you can take them off and turn the little knob by hand to see if you get heat when turned. it dont move much, so dont go trying to make it turn more then a 1/2 inch either way.