Joe,
It looks like Whirlpool forgot to include the defrost thermostat in the parts breakdown diagram for your model. If you look at the old thermostat, you should see a temperature stamped into the side of it... it will look something like this L42-30F in very small letters. This means the thermostat shuts off at 42 degrees and kicks back in after it cools down 30 degrees. If yours is a different temperature than that please let me know, but most of these style refrigerators use the same style thermostat.
Defrost Thermostat (L42-30F) - W10165425