My 2002 Forester heater will only emit weak hot air, but will not be it enough to keep car warm

The rear defroster in the rear windshield and weak heat from the front vents may not be related. For the low heat issue you need to start with checking the coolant level when the engine is cold. If that is ok then feel the two hoses going to the heater core. They need to both be equally as hot. Alex