Could be defrost heater, defrost thermostat or defrost timer(main module)
Each heater should show continuity with ohm meter. If one of them looks black(burned) it's bad.
Thermostat is clamped to the condenser coils. You can bypass it to check the rest of the defrost system. It breaks the circuit when it is warm and completes the circuit when cold.
If these are good it's your control panel. http://www.searspartsdirect.com/part...68513/0046/464