The inertia fuel shutoff (IFS) switch (Figure 8) is used in conjunction with the electric fuel pump. The purpose of the IFS is to shut off the fuel pump if a collision occurs. It consists of a steel ball held in place by a magnet. When a sharp impact occurs, the ball breaks loose from the magnet,rolls up a conical ramp and strikes a target plate which opens the electrical contacts of the switch and shuts off the electric fuel pump. Once the switch is open, it must be manually reset before restarting the vehicle.
The location is shown in the owner's manual. If you do not have one getting one will give you a lot of information about your Windstar.
I found this on the net for 99 to 03 vehicles
The switch is located inside the jack storage compartment found at the right rear corner of the vehicle. Remove the cover and it is behind the thin "foam" covering inside the compartment towards the rear of the vehicle.