Hi Mark,
Looks like you were right and the first failure I suspect must be the defrost thermostat (#35 on the
unit break down diagram). If it stays closed during the full defrost cycle (which it's not supposed to) the defrost heater stays "on" to cause excessive heat in the freezer.
- The part number for the defrost thermostat is
AP4299920
Here are the break down diagrams for the Whirlpool refrigerator
Model ED20TKXFW00
Gene.