Replaced defrost heater harness.
The lower heating element had burned out.
First, I had to melt the solid cake of ice buildup off of the evaporator coils behind the back panel in the freezer unit.
Used a hair dryer to do this. Works very efficiently.
The replacement defrost heater harness came as a complete unit that includes both upper and lower heating elements as well as the thermostat.
Very easy to replace.
The fridge is almost 20 years old, but the replacement parts were identical to those removed.