2000 Chrysler 300 no hot air

Friend's car 180,000 miles. The antifreeze was orange as in rusty/corroded blows cool air. Took it for a radiator flush worked okay for a few days. Now it's hit and miss car warms up to normal operating temperature. Sometimes it blows warm air sometimes not. Could this be the heater core ? Doesn't sound like a thermostat but where is it located in radiator ? Or in the intake manifold. Any help is much appreciated, don't have access to a garage until the weekend.