We need a little more info but typically rear ac not working is due to a bad fan or possibly loss of refrigerant.
If the rear air conditioning is not working, this could be due to one of severalÊproblems. It could be due to a blown fuse.
If you are talking about dual heating (front/rear), then either the rear temp control is set to cold, or the control is not working.
This could be a multitude of answers as you didn't say if the car is almost dragging the rear bumper or has the stinkbug look. If the rear is down it could be the bag or bags are leaking, sensor is bad or compressor has failed. If the rear is high in the air then the valve is stuck so the excess air is not bleeding off.
you will hear it run
The rear air conditioner unit has a thermostat that is malfunctioning and needs replaced.
If it has air bag suspension, air bags could be shot or the air compressor for the air bag suspension not working.
check the air lines, there can be holes or cracks in the bags or the compressor is not working
the front air shocks are bad
Check the temperature control valve. You're probably allowing hot engine coolant through the rear heater coil.
Its a faulty rear control switch (a/c or heat) on the dashboard. It works by vacuum and when it doesn't work correctly it defaults the rear heat. I just had this repaired on my '98.
You probably have a a/c leak.
Several things could cause the Ford vans rear air to not blow. There could be a blown fuse to the blower motor. The fan control could be bad, or the motor could be failing. Also there could be a wiring problem, or the hose from the blower to the vents could have a hole in it, diverting the air.