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Of course it could just be a vacuum problem going to the rear. You should be so lucky...
The rear HVAC on your van is not a blend door type. It is an either/or type. You have no blend door or actuator, you have either heater or A/C. What we really have is two separate systems that share a common blower motor. When the blower is directed rearward, you have A/C out the top. When the blower is directed forward, you have heat out the bottom. The sad news is that the vacuum operated selector door actuator just loves to break. And its a pain to get access to. Most/all of the paneling on the side needs to come off to access the rear HVAC. Once opened up, you will see 2 separate systems, divided by a vacuum operated door. When it breaks, it defaults to heat. To access the rear HVAC unit you will need room. Pull out the middle and rear seats. Remove the plastic side facial pieces starting from the top, then work your way down. Once the last big piece of plastic comes out, you will see your HVAC unit. Look for a vacuum actuator and test for vacuum with rear A/C requested.
Of course it could just be a vacuum problem going to the rear. You should be so lucky...