1999 Ford Windstar Heater Control Valve

you probly have a broken vaccum under the hood near the heater core it is a red line very common

The most common problem to cause loss of heat or A/C in Ford Windstars is broken blend doors. The blend doors are located inside the plenum box, which sits underneath the dash near the glovebox. In order to access/replace the blend doors, the dash must be disassembled and the plenum box extracted. There is also some methodology popping up online that allows you to access the blend doors by removing the glovebox and making a cut in the plenum box, it is much less labor and time. Here is more info about the problem.