Question is, do you live in such a location that it gets cold and you really require a working rear defogger?
If you do live in such a place, then the best remedy is to replace the complete rear windshield and get a new original one.
Or, retain the existing rear windshield and use the heating ventilation system (HVAC) with air intake set on 'Fresh'. In case of a rainy day, set the heating on maximum or whatever you are comfortable with and switch on the A/C. Yes, the air conditioner. The A/C will dry the air and the heater will heat the air and once it enters the car it will clear all the condensation on the glass surfaces including the rear windshield.
Hope all this advice helps. Final decision is yours!