Because the rear window heater is electric. Your heating is based on a small radiator behind the dash so get that checked
Andy C
The rear defrost is electric. The car heating system is hot water from your radiator
The defrost in the back glass is done electronically with wires that get warm with electricity. The heat issue could be several things, heater hose issue, heater core, blower motor, and a few other things. Does the blower fan work just no heat? Answer in comment section please.
The rear window uses a heating element on/in the glass that just needs power to heat up.
you may need a new blower motor resistor
