I have No heat after a coolant change...

[Two560Two]

I just acquired a 91 560sel with 124k miles. It seems as if the PO just recently did a collant change to zerex? Well, I have no heat! The engine is at full operating temp but the heat doesnt come through. Only cool air blows through. The A/C works fine and was able to test that today since it was 60 deg. Now its a bout 39 deg and I get no heat. I ran the defrost on high thinking there was air in the system but still no luck. I did that for about 5 minutes though. I am thinking that i need a new heater core? There are no leaks or coolant anywhere. Please help