Remote rear heater box not working properly

I have a 1999 Express 1500 conversion van. The dash heat and A/C work fine. But, the remote rear heater box blows cold air through the rear vents (on all dash heat settings). Is there some reason why hot radiator fluid does not reach the rear unit? Is there some switch that prevents it from flowing to the rear unit? If so, where is that switch located?
Thank you,
John