The ignition coil on a 1998 Lincoln Navigator is located
on top of the engine, near the distributor. Since the Navigator uses a distributorless ignition system (DIS) with individual coils per cylinder, you'll likely find multiple ignition coil packs mounted directly on the engine's valve covers. There isn't one single coil.
To be precise, you'll need to consult a repair manual specific to your 1998 Lincoln Navigator's engine (likely a 4.6L V8). A diagram showing the location of the ignition coils will be very helpful. You can often find these manuals online or at auto parts stores.
