One possibility is the blend door insude the blower plenum behind the
glovebox.  There is a servo motor control on top of the plastic air
handler plenum box.  A b**** to get the control out due to a mounting
screw on the back side.   You can remove the control and with the
system running watch the output lug rotate when you change temperature
settings.  If the control is bad replace it.  If the control works then
check the mating socket on the blend door.  I suspect it is broken;
that the servo motor is unable to open the door and allow airflow
across the heater core.  (my door is broken and I have yet to tear into
the system.  Plan to do so in the next couple weeks as cold weather is
beginning.)  The trick to removing the rear screw on the servo is to
pop the trim at the base of the windshield and use a long extension to
go down to the screw guiding the socket with your fingers at the screw
head which you can feel with finger tips.  Toss the screw for
reassembly as it isn't necessary the way the control mounts to the
plenum the two screws on the glovebox side are adequate.

Sounds like new thermostat is stuck open... 'tis a bummer but it
happens.

HEATER CONTROL VALVE?8:12 -0400, "rdome43417"