Hello:
It is very common to have the line in the freezer door freeze up. I would start there first. If that's not it then next step would be your water valve which is located in the back/bottom of the fridge. It is a dual valve( one side for ice maker the other for water dispenser). you can check that by taking off the front kick plate and on the bottom freezer door hinge you will see the line entering the door. Follow it towards the under the fridge and you will find a coupling. Disconnect it and see if there is any water coming out when you press actuator on the door .. If yes then its frozen if no your valve is Kaput.
Cheers
Your defrost is controlled by main control board located in the back of the fridge. I would tho check continuity on the defrost thermostat(cuts off power to defrost heaters when it gets too hot). If the t-stat is bad it will stop any voltage getting to the heater. Its located on the evaporator coil and you can only tested when its temp is below 30 degrees..
chck out this video
First run a fault code test at the diagnostic link under the dash on drivers side and see if any fault codes are in memory, if nothing shows then test the fuel pressure, the test port is in the fuel supply pipe that sits over the fuel injectors on the top of the engine, looks like a large tire valve. You will need a special tool, borrow one from a local auto parts store like Autozone.
