To be certain, you should:
* Consult your owner's manual: This is the best resource for the exact location for your specific year and model.
* Search online forums: Forums dedicated to Lincoln Navigators (like those on Navigator specific sites or general automotive forums) will have discussions and possibly pictures showing the compressor location. Search using the year and model of your Navigator.
Because it's under the vehicle and can be difficult to spot, a flashlight and possibly a creeper (a mechanic's tool for working under cars) may be helpful to find it.