is hot only throught the defrost im guessing the system the system was bled? inside the cabin is the heater core. The engine warms up coolant and when you turn the knob to heat the air inside, coolant flows through. Outside air flows across this radiator and blows heat onto you. When you drain the coolant system you also drain the heatercore. When you fill the coolant back up, a large air bubble is trapped in the heater core. When you turn on the heat inside, that air bubble goes out and into the engine. I would check your coolant level immediately and bleed the system.
When bleeding the system, turn the heat on full-blast (high temp) while you let air escape from the bleeder screw.