I have replace my defrost heater two times already. But they seems to keep blowing out. What is causing this to happen? Is it something else wrong or the parts where defected.

One or all of these components may be faulty.

1. Defrost heater faulty.
2. Defrost limit switch faulty.
3. Defrost timer faulty.

You need to fully test the other components before replacing the defrost heater element.