First check the coolant level. No heater will produce heat if the
coolant is low. Also check for an air bubble in the system.
My guess is your Subie has consumed or leaked enough coolant to put a
air pocket in it. is the res empty? open the rad cap, Is fluid up to
the top? yes, then disregard the following. Add coolant to the res,
fill the rad full and cap. idle car with it off both defrost settings
until A: hot air blows from the blower, b: the cooling fans kick on by
themselves. You may need to hold the rpm's at 2000-3000 to get the
fans to turn on. When the fans are on, hot air blowing, and the gauge
is stable at slightly below 1/2 then you turn off the car, turn the
key on and the ac on- this will turn the fans on and cool it, drawing
more coolant from the res into the rad.