GE Refrigerator Defrost Problem in the Freezer

GE Refrigerator (Model#GSS22KGMB)
Here is a basic photo of a evaporator coil with massive frost build-up, but what part has failed?
Good Question, The first thing you do is move the defrost timer into defrost, if the defrost heater does not come on, then you know for sure that’s its either a bad heater or defrost thermostat. You would need to get a ohm meter to check the heater for continuity.
(With power off-refrigerator unplugged) the resistance of the heating element – it should measure
under 100 ohms (31 ohms typical). If it reads this then the heater is good. If not, you would need to replace the heater assembly.