1987 Lincoln Town Car.  When its cold outside, the heater usually comes
on. But about 20% of the time, heater wont come on.   Its not the fan
motor because it works on all 4 speeds when the system works.
Also i can feel heat radiating through the vents, just no fan to push
the air.  So hot water is in the heater core, but the fan wont come on
to push the air.  I can move the setting from heat, to vent, to aircond,
and the fan still wont come on.

There is a sensor/valve in the heater hose that wont turn let the blower
come on until the water reaches 120 degrees.

Or could it be the thermostat inside the car that determines the amount
of heat.  On this car you can set the desired heat much like using your
thermostat at home.
If it is the thermostat inside the car, where is it and how do u get to
it. Hope i don't have to remove dash.