Auxiliary Water Pump
This Auxiliary water pump often goes bad with age and is relatively easy to replace. It is located on the firewall, inside the engine bay, behind the fuel filter. It is approximately the size of a can of soda.
You can test it very simply by turning your van off and pushing the "REST" button on the far bottom right of the climate controls. Two things should happen when you push the REST button: The fan should turn on to low speed and this Auxiliary pump should start pumping. The engine is OFF, so you can usually hear the pump humming or you can put your hand on it and feel it vibrating. If its not pumping, you can pull the two wire plug off of it and test for voltage. If there's voltage but the pump isn't running, you need a new pump. If there's no voltage, your climate control module/head unit is faulty.