My 1999 Ford Taurus is not blowing hot air through the heater or defrost what are some suggestions?

When I bought the car it was over heating. The service engine light was on and cylinder 1 was miss fireing. I had did everything else so I put some block sealer in the coolent and it has ran hot or cut out since. So it was a head gasket but I'm wondering if the sealer might have clogged up the heater core. What are some other things to check or to do before I replace it or how do I know if it is the heater core?