Refrigerator NOT Defrosting

Had to use a hair dryer to manually defrost the refrigerator before I could replace the heater assembly . I had to remove the aluminum mounting brackets and reverse them before mounting the assembly so that the wiring would run the same as the old assembly . Overall time spent from start to finish, including manual defrost was about 2 hours.
Thanks for the timer troubleshooting tips, they helped to narrow down the problem.