I am not quite sure that you’ve got the correct method of operation there. When my cable for building a hot callus pipe arrived earlier this week I wanted to test it right away and make sure that the wire functioned correctly. Just for giggles I plugged it in to an outlet inside the heated house and let it run for about an hour. It was still cool to the touch. Then I unplugged it and I took it out to the garage. I placed the thermostat portion (that which is circled in white)
inside of the freezer section of the fridge/freezer in the garage, and then plugged the heating cable into an extension cord. About an hour later I came back and picked up the long black portion of the wire that was outside of the freezer and it was amply warm, confirming for me that the thermostat is in that overmolded portion of the wire.
If you have any question, feel free to look over at another thread that confirms this function: