My '87 Caravan had it and my '91 Caravan has it too. The drain that is supposed to allow water in the heater to run out thru the firewall has a plastic plug (Carplug) in it, that was apparently not removed when the vehicle was assembled. On the '87, it caused the water to sit in the heater and rust out the bottom end of the hinge on the heat flap, making it necessary to remove the whole heater from the vehicle to fix it.
If you look at your firewall and see the drain tube has a plug in it, you may want to remove it.
Is there any reason why it should be there??
'Curly'