IMPORTANT: Defrost heater was more difficult to install around the evaporator coils than the APP installation videos show, but it can be installed. See below.
FIRST: Unplug the fridge/freezer!
I tested the thermostat, mechanical defrost timer, and the defrost heater with a ohmmeter and determined that the heater was bad.
NOTE: Thermostat must be cold in order to be tested; otherwise, the switch inside won't close and you won't get a reading from the ohmmeter. Take a can of compressed air, hold it upside down, and spray the thermostat. The propellant escaping from the can will quickly freeze the part for testing.
Defrost timer testing is too complicated to describe here; see this video: https://www.youtube.com/watch?v=tPpLl2d0094
Testing the heating element is simply a matter of testing for continuity. Unplug it from the wiring harness and test it while it’s still installed in the freezer.
As for the installation of the heating element, APP’s videos all show a “U” shaped heating element that can be slid in between the evaporator coils from the bottom. This was not the case for me because the top 4 inches of the new heating element was bent inward (see part #218657301). I checked with APP support and verified that while the heating element can be bent *slightly*, I decided against trying to straighten out the element completely rather than risk breaking the part. I bent the part just a little by gently pulling apart the legs of the heater, and tried to straighten one of the top bends by only 2 or 3 degrees of angle.
When I installed the heating element, I took the straighter of the two legs and fished it up through the right side of the evaporator coil about 6 inches or so until I was past the first bend in the heating element. This is crucial because the bottom of the evaporator coil on the right is closed off by a loop of tubing and the element must be fished through. The left side doesn’t have this problem. Once I got past that point, I was able to slide the element up while rotating the other leg of the element to the left so it could clear the left side of the evaporator coil. I managed to then nest the left leg between the evaporator coils on the left side and then position the heating element into place.
Hopefully, this will save someone else the aggravation of trying to figure this out all over again.