If the heater ohms out fine and you have continuity and you have replaced the defrost thermostat then the only other issue would be your defrost control. Depending on the age of the unit it may just have a timer style defrost control or an adaptive control board. The thermostat would be located at the top of the evaporator coils. Defrost controls are probably the least likely of the system to fail. Usually when they do the unit sticks in defrost and the unit will be totally dead. If its a dial style timer you can advance it with a flat head screwdriver until it clicks this should send power to the defrost heater. Hope this helps _MJ_