The Idle Air Control (IAC) valve on a 1999 Ford Expedition is located on the throttle body. It's a small, usually rectangular, electrical component mounted directly to the throttle body itself. You'll likely find it on the side of the throttle body, opposite the throttle cable.
To be certain, you'll need to consult a repair manual specific to your 1999 Ford Expedition's engine (4.6L or 5.4L). A diagram will show the exact location. You can often find these manuals online or at auto parts stores.
