Make sure the cooling tank is full to the mark as mentioned and refill as nesassary... After the van has been warmed up to operation temp, you should be able to feel the upper radiator hose to see if you got a stuck closed thermostat... If the hose is really hot then it's working, if not, bad thermostat... I would just say replace the thermostat anyways but it's looks like a pretty tough job all for putting an aftermarket crap thermostat, avaialble at these parts stores. So with that outta the way and im sure it still doesn't work, underneath the van, behind the drivers side rear tire you should see about 4 lines going to the rear unit (hopefully connected)... With van still running and rear heater turned on, 2 of those lines are heater lines and they are a bit bigger than the 2 smaller A/C lines and they should be pretty hot to the touch... If they're hot, then it's probably a rear blend door problem perhaps a busted vaccum line or actuator, if those lines are not hot, im sure it's a heater control valve issue which again is vaccum operated and could be a busted line or valve... Clogged line is doubtful but could be checked by feeling one line at a time all they way up to the front feeling for hot and cold spots. | 
