heater only works on my 2002 v6 grand am when its being driven but blows cold air while idling.. what would be the problem

Face uphill, park with the cap higher than the engine, run the engine, add 50% type coolant. get it full. Continue diagnoisis to attempt to reverify concern.

Leak test and inspect for leaks, inspect waterpump housing at shaft behind pulley, tighten connections and verify maintenance is up to date.

Performance test heating and ventilation system to attempt to verify any concern still exists. Define any issue by operating controls, report.

i would check the thermostat and thats almost sounds like the beginning of a head gasket problem. you need to flush the coolant system and then bleed it and see if that helps.