Your fridge has the infamous motherboard in the back. Remove cover, you will see a bib motherboard. Turn power off and look in the left hand corner Locate pins J11 and J9 Jumper these pins together and measure defrost current. If no current, you have either a bad defrost thermostat or defrost heater, just replace both. If current, you will need to replace the motherboard and the evaporator thermistor. | 
