Sounds like you have got it figured out. If you don't ready any continuity through the defrost heater (AP2071520) (with it unplugged is the best way to check) then it is bad. You also verified the thermostat properly. As for the timer, the best thing to do is turn it until it clicks, then verify voltage to the defrost heater. No voltage, most likely bad timer. If you do have voltage, then the problem is with the defrost section. There is also a defrost thermostat (AP2071271) that can cause a defrost problem and you simply check it for continuity also. Just be aware, it should only show closed when at normal freezer temperatures. If you check it when everything is iced over, you should get an accurate reading. But based on what you have provided, it sounds like you need a defrost heater.