Defroster not working since having radiator replaced

Does the heater work? What is not working? No air flow or no heat? Did you return to the shop to have them look at it?

If air flow is normal but the air temperature is low from both the heat and defrost settings, suspect an air pocket in the cooling system that hasn't burped out yet. In a worst case scenario, that could lead to engine overheating and a warped cylinder head / leaking head gasket. ($$$)

Check the coolant level in the radiator, (not just the reservoir), after the engine has cooled down.

Caradiodoc