There is no separate defrost timer as such.
The defrost timing cycle in your refrigerator is initiated and controlled by the control board.
Usually with auto defrost models, the defrost cycle occurs once every ~10 hours and lasts for about 25 minutes. During this time the compressor and the evaporator fan are both turned off.
The defrost heater (part #WR51X10038) is turned on to melt the ice build up on the evaporator unit and the ice meltwater drains away via the drain tube to the evaporator pan underneath the refrigerator - near the compressor motor.
The defrost thermostat (part #WR50X10090) controls the power to the defrost heater because you don't want to let the evaporator unit get too hot, as it is only thin Aluminium and is designed to operate at below freezing temperatures i.e. 0°F.
The defrost thermostat operates at 55°F and cuts off the power from the control board to the heater.
Once the defrost cycle time period is finished, the control board, removes the power to the heater circuit and restarts the compressor and evaporator fan to drive the temperatures back down to the set temperatures. When the temp in the freezer compartment reaches 32°F the defrost thermostat releases in readiness for the next defrost cycle.
I'm not quite sure about the resistance value of the defrost heater in your model but most are somewhere between 20-50 Ohms.
If you measure the heater directly with an Ohmmeter and get a reading of OL or open circuit, the heater is faulty.
The parts linked above are for your model.
The supplier was only to get the manufacturer's part number. If you search online using the manufacturer's part number only you will find other suppliers that may suit you better.
If both the defrost thermostat and the heater are OK and if there is power going to the heater during the defrost cycle, check that the drain tube is not blocked. If the drain tube becomes blocked for some reason (check at both ends), then the meltwater cannot drain away and it will build up/refreeze etc with every defrost cycle to a point where the defrost heater cannot overcome the amount of ice and it will ice over the evaporator fan and stop it from working. The first that you know about this is that the refrigerator is warm but the freezer is cold.