Starter Solenoid
WARNING: This page is about a different car, the 2004 Ford Econoline. However, it is still accessible from the selected car via links, so may be relevant.
Remove starter from vehicle. See STARTER MOTOR under REMOVAL & INSTALLATION. Check resistance between starter motor ground terminal and solenoid case. Resistance should be about .95 ohm. If resistance is not about .95 ohm, replace starter solenoid.