On a 1999 Ford Expedition (assuming the 4.6L V8 engine, which was the most common), the coil packs are located
on top of the engine, directly above the spark plugs. They are usually grouped together and are fairly easy to spot. They're typically black plastic rectangular or square units.
To be absolutely certain, a visual check with the hood open is recommended. You can also consult your owner's manual or an online repair manual with diagrams specific to your vehicle's year and engine.
