These units are a pain in the neck to get to the defrost heater. To do so without breaking any of the plastic panels or the syrofoam divider block, you should turn the unit completely off for 24 hours with the doors open to let the unit completely defrost.
First, remove the freezer door.
Remove the middle hinge, remove all the screws in the divider between the freezer compartment and the refigerator compartment.
Gently pry the plastic divider (also known as the mullion strip) towards you.
Pull the floor of the freezer out (very gently) as these are known to crack.
Remove the styrofoam divider under the freezer floor.
You will find the glass tube defrost heater clipped under the evaporator with a couple clips.
Simply remove the clips noting their position on the evaporator and unplug and remove the glass tube defrost heater.
If either the defrost heater or the bi-metal are bad
it would be well worth the few extra bucks to replace them
both so you hopfully never have to go into the unit again.
Defrost bi-metal Link
Source: Montgomery Wards, Refrigerator HMG19274