1998 Ford Windstar van

You have rear heater system !
Check hoses and lines
rear heater core

This is some pictures

I am sorry
Let me see and try to explain a little more

for the heat to work in the rear you need the engine coolant to run back to the rear where they have installed a heating and AC unit
1st need to inspect and make sure that all you lines are good and not rusty causing the leak
and if they check OK (visual check )
and the leak is coming from inside where the heater core, then you have to remove the inner trim to check and replace if needed
determine where the leak coming from and if need info on how to remove and replace the heater core I will try and get you the steps needed

if cannot locate the leak for sure, then take it to a shop and let them pressure test the coolant system for you