I assume you have dual zone control in your van!? Either way both blend door motors are located on the left side of the heater box, beside the gas pedal. You'll have to remove the silencer pad from beneath the instrument panel and remove the three screws that secure the heater core shield to the heater box. Once removed you'll have a clear view of the motors and can remove them and manually move the door to see if it makes a difference in the temp. I have seen a non-dual zone have a block in the evaporator core causing a warm and cold air split like you are describing. Replacing the evap fixed the car,just something to keep in mind.