Start by feeling the two hoses running to the heater core. If both or very hot and equal in temperature, a bad temperature door actuator is likely. This actuator is to the left of center, under the dash and controls the blend door.
If one hose is cooler than the other, a plugged heater core is very common. We have had good luck, flushing these cores, but you must be VERY careful. The antifreeze odor seems more like a leaking heater core. This can occur if the coolant is bad enough to cause plugging of the core. Here, heater core replacement would be the choice.