1998 Ford Windstar Heating Issues

I had the same problem with my 98 windstar. It was caused by low, very low coolant level. I would fill the radiator, and the overflow tank and the heat worked great for a week or two, until it lost all the coolant again. Mine turned out to be a clogged radiator that was showing normal coolant temp, but was letting the coolant boil out while I was driving (never saw coolant under the van). Try topping off the radiator and the overflow and take it for a drive. You should have heat. If it loses all the coolant again I would recommend replacing the radiator (mine was 99$ at Autozone).