Dont know what kit you have but when I installed auxillary drain heaters (non factory) I had to heat the drain area up and use foil tape to secure the heater onto the pan. Heating with a heat gun (hair dryer) was necessary otherwise the tape would not stick. Drain heaters I used were skinny braided heaters with different wattages for different applications. a small 10-30 watt heater should suffice. I wired in parallel with defrost heater and installed a small portion of it into the drain hole itself.

My kit did include instructions, not real clear buy adequate. The new freezer block did slide in with some difficulty, it may have been the way I installed the tape. Anyway, time will tell if it works or not. A new problem arose and now the refrigerator maintains a temperature close to 30F, too cold.