Unstop a stopped up heater core

Install a new one. You could try a radiator flush but if it is stopped up it is time for a new one.

FOR is probably right. But one thing i have done is reverse flushed just the heater core. Find the inlet and the outlet for the lines going through the firewall. Figure out which is which then disconnect them both and put the garden hose up to the OUT heater core line. When i did this it worked...but a few weeks later the heater core started dripping all inside the car and I had to replace it anyway. Ha ha ha ha. oh well.