Heater Core Replaced, Now Blows Cold Air

Like Krusty said, very likely an air pocket is not allowing circulation through the heater core. Touch both hoses where they enter the box, after the engine reaches operating temperature, if only one is hot , apply vacuum to the nipple where the green wire is connected. Oh, that's a hose not a wire. If the hoses still aren't hot to the touch, bring it to a shop that can bleed the air from your cooling system. Also make sure your water pump is circulating the coolant.
Good luck