You could have air in the system, you have a faulty blend door, which is acting erratically, or you have a failed thermostat, where it is stuck open. But I would check this first... It may be something as simple as not enough coolant or water in the cooling system - reservoir or radiator.
if there is no fluid in the cooling system, there is nothing to get hot, therefore you will have no hot air blowing into the cabin.
also, as an Astro owner, check out this link...
http://astrosafarivans.org/bb2/viewforum.php?f=54
lots of useful and helpful information, from its members, just like here at FixYa
good luck

You have a heater control door stuck in the closed position. This may require removing the dash to gain access. Possibly due to some item being dropped down the defrost vents. I am not sure on your car what is required to fix.
You may be able to access the heater controls by removing the dash controls and checking the vacuum lines at that point

Vaccume controls the heat gates ie. defrost,floor,ect.Your thermostat controls engine temp.try a new thermostat.Also if your engine temp gauge reads normal and still have no heat you might have air bound in system.Vans with aux. or rear heaters are pron to this .You flush it but did you bleed the system?

Heat sensor for radiator fan switch, thermistat,water pump? Does hot air blow when moving? does car over heat when sitting idling for over 10 minutes. If so check fan first, then water pump. last cause thermisat