The defrost heater element looked good and had continuity. The defrost thermostats checked good, so that just left the Defrost Module as the likely suspect. I installed a new defrost control module and thermostats. The thermostats were inexpensive and the old ones could have been intermittent,
The only "tricky" part was finding out how to get to the back screw that holds the main control panel. Discovered that the return air duct surrounding the control panel needed to come off. It helps to have a long, narrow screwdriver to unlatch the duct, when the front edge of the control panel is down.