The rear heater is interlocked with the defrost setting. When in
Defrost the rear heater BLOWER does not work.
There can be problems with the coolant lines that go to the rear
heater. To replace it per the factory book you have to remove LOTS of
stuff. Many owners have made their own repairs. It is also possible
that some one just re-routed the hoses to avoid a leak.
The third possibility is that the electrical valve that controls the
coolant flow to the rear heater is not working.
