I have some information from the 1998 model of your van, but this should be pretty close to the same as yours.
Here is the description of the rear heater:
And here is the schematic:
Here is a diagram of the valve location I found.
You will notice the "Heater Control Valve Solenoid" in the lower left of the schematic. What could be happening is that the valve has gone bad. You COULD just bypass that valve, until you get a replacement, by putting a straight piece of pipe in there.
What I would do is check that there is power and ground at the terminals of the connector for the valve when the ignition is switched to RUN (engine off) and the blower motor for the rear heat is set to LOW, Medium, or High. Then switch the blower to OFF and see if power "goes away". You might want to ask someone to help you with this, so you can hear or feel if the valve & solenoid is clicking or moving when switched from OFF to one of the other settings.
Once you check that the voltage is switching properly, you should start the engine and allow it to warm up, then switch the blower on and see if the hoses get hot, past that valve, after a minute or two. If not, the valve will probably need to be replaced.
I also checked at rockauto.com, and they show that the valve should look something like this:
Please let me know what you find.
Take Care, Greg A.
P.S. I almost forgot - you might want to take the hoses loose that run to the back heater core and run water through the rear core system, in both directions, to make sure that you have good flow to the rear core.