The coolant is a tad low, just barely. Shouldn't cause any problems like that. And the rear heater seems to be working fine. There is a rear heater control for the back passengers. Idunno...
Why won't my front heater work? on 2002 Ford Windstar
check the blend door motor. it is a common failure. you may hear a clicking noise behind the dash. it is the motor trying to turn the door to heat.
1 more answer
Have a good technician check the operation of the heater blend door, that controls how much heat that you get.