I just bought my son a 2001 Chevy Blazer and the heater core was leaking, so we changed it out, but now the heater is only blowing cold air. what did we do wrong?

 

Why is My Car Heater Blowing Cold Air?

 

Try flushing the heater core out. There may very well be air trapped inside it.

 

after flushing the heater core, how do I make sure there is no air in it?

 

there are vacuum tubes which open and close valves to let hot air thru the vents. they are located drivers side near the fire wall