1. Verify the Fuse: Even though you think it's not blown, visually inspect the fuse *carefully*. Sometimes a hairline crack or internal break isn't immediately obvious. Replace it with a known good 20-amp fuse just to be sure.
2. Check the Wiring:
* Fuse Block Connections: Examine the connections at both ends of the fuse – both where it plugs into the fuse block and the wires connected to the fuse block itself. Look for corrosion, loose connections, or broken wires. Wiggle the wires gently to see if there's a loose connection.
* Wiring Harness: Trace the wiring from the fuse block to the trailer connector. Look for any damage, chafing, or breaks in the wiring harness. Pay close attention to areas where the harness might rub against the frame or other components. This is a common point of failure.
* Trailer Connector: Inspect the trailer connector itself. Make sure the pins are clean and not corroded. Try a different trailer connector if possible to eliminate it as a source of the problem.
3. Test for Power at the Source and Destination:
* Power at the Fuse Block: Use a test light or multimeter to check for power *at the fuse block* on both sides of the fuse. You should have power on one side (the input) and no power on the other side (the output) if the circuit is open. This will confirm if the problem is before or after the fuse.
* Power at the Trailer Connector: With the truck's ignition on, check for power at the trailer connector pins that control the stop and turn signals. A multimeter is best for this, as a test light might not be sensitive enough for low voltage signals. Compare the voltage to the power supply side.
4. Consider the Tow Package Wiring:
* Relay: Your truck likely uses a relay to switch the power for the trailer lights. The relay itself could have failed. This is often located near the fuse block.
* Wiring Diagram: Obtain a wiring diagram specific to your 1998 Ford F150's towing package. This will show you the exact path of the wiring and help pinpoint the problem area. You can often find these online through repair manuals or Ford forums.
5. Ground Connection: A poor ground connection can prevent the lights from working. Check the ground points associated with the trailer wiring harness.
Important Note: If you're not comfortable working with automotive electrical systems, it's best to take your truck to a qualified mechanic. Improper wiring can lead to further damage or even electrical shorts. Incorrectly working tail lights can be a significant safety hazard.
