Where is the air condition compressor located on a 1999 Ford Expedition?

On a 1999 Ford Expedition, the A/C compressor is typically located on the front of the engine, driven by the serpentine belt. It's usually near the passenger side (right side as you're facing the engine), but to be absolutely certain, you'll need to look at a diagram specific to that year and model. A repair manual or an online parts diagram (many are available for free with a little searching using the engine size as well as the year and model) will show you the exact location. Looking at pictures online for "1999 Ford Expedition AC Compressor" might also help.