1999 Plymouth Breeze

Do you mean blocking the air flow or it's blowing cold air?

If the air is cold, and assuming the temperature gauge reads normally, feel the two heater hoses. They should be too hot to hold onto for very long. If not, suspect a plugged heater core. It can be back-flushed with a garden hose.

If it's an air flow issue, can you hear the fan running? Does the air blow from the defroster outlet?

Caradiodoc