If by "heater box" you're referring to the heater core, we've got you covered with the "Related Questions" below.
Be sure to review the questions regarding the blend door and blend door actuator motor - other very popular heating/cooling parts in the Windstar
How do I replace the heater core in a 1999 Ford e150 conversion van
electric window binds on 2003 ford windstar
The most common reason the heater is blows cold air on a 1999 Ford Winstar is a bad thermostat or blocked heater core.
The CD changer is in the center console of a 1999 Ford Windstar
Replace freeze plug in a 1998 ford windstar ?
In a 1999 Ford Windstar, the heater fuse is typically located in the fuse box under the dashboard on the driver's side. You can access the fuse box by removing the cover, and the specific fuse for the heater is usually labeled on the diagram inside the cover. If you're having trouble locating it, consult the owner's manual for detailed information.
There is no MAP sensor on a 1999 Windstar.
how to drain 1999 windstar
how do you reset the anti-theft system on a 1999 Ford windstar
The "Related Question" below is an extensive discussion on the 1999-2003 temperature control valve / heater control valve. It is likely the information will be similar to the 1997 / 1998 Windstar
heater windstar ford
how do you reset the anti-theft system on a 1999 ford windstar