The fan in the freezer compartment only comes on for a few seconds and

Hi @michaellin65785

The evaporator fan (inside the freezer compartment) is controlled by the temperature control thermostat and the defrost timer, as can be seen in the wiring diagram for the model.

There should be 120V AC at the evaporator fan, if the compressor and condenser fan are operating, regardless of whether the doors are open or shut.

If not then there may be a wiring problem somewhere, especially if the fan operates with the freezer door open but not when it's shut.

The only time when there isn't 120V AC at the evaporator fan is when the temperature control thermostat has operated, indicating that the set temperature has been reached in the compartments, but then the compressor and condenser fan won't have any power either and they'll be stopped as well.

What other problems are there that makes you want to locate the defrost thermostat?

The defrost thermostat is used to disconnect the defrost heater if the temperature near the evaporator unit in the freezer goes above 46°F, during the auto defrost cycle, thereby preventing any damage occurring to the evap unit. If the defrost thermostat is faulty i.e. open circuit, the defrost heater won't work during the defrost cycle which will eventually cause the evap unit to become covered in ice. This can also prevent the evap fan from operating as well.

The defrost thermostat¹ (example only to show what part looks like) is part #18 in this freezer section parts diagram for the model. It is usually either mounted on the evap unit, or clipped to the pipes connected to the evap unit.

¹ If the defrost thermostat is faulty, it is still available. Search online for 63001599 (supplier examples only) to find suppliers that suit you best.