Why do the heater work and the defrost don't work?

If the heater works but the defrost does not, the issue could be due to a malfunctioning blend door or actuator that controls airflow between the heater and defrost settings. Another possibility is a blockage in the defrost duct or a problem with the climate control system. Additionally, low refrigerant levels or a failing compressor in vehicles with climate control systems could also affect defrost functionality. It's best to have a qualified technician diagnose the specific problem.