1999 Chevrolet Malibou: Good engine temp. Full coolant system. Inlet hose to heater core hot. Discharge hose to heater core is cold. Analysis: Core is plugged or dirty and needs flushed?
there is a valve in the inlet heater hose. this valve shuts off the water to the heater core when the AC is on. make sure it is not stuck in the OFF position. sometimes tapping it will dislodge it enough to gewt it to open. then replace it, when you are sure. cause it WILL stick again.
if that valve is OFF, then NO water can get to the heater core.
this valve is an electronicly actuated valve. not sure what it looks like on your car, but do some digging, you’ll find it.