Hello. I have a 2004 Ford F-150 lariat with a bit of a problem. My running lights (not sure if correct term, I'm talking about the tail lights' illumination that stays on while you drive at night) are not working. My headlights and parking lights do work, and my brake lights and turning signals also work. I have put in a brand new brake light switch and checked every fuse and relay, but no luck. Any advice?

 

Here's a diagram of the taillights for your truck.

 

"A" in the above diagram comes from the "A" in this diagram, which comes directly from the switch.

 

Note in the first diagram the license plate lights are fed from the same wire. Do they work?

 

Parking lights = "Tail lights" or "Running Lamps". All the same thing.

The tail lamp circuit is carried to the rear of the truck via a wiring harness that runs down the left frame rail. Look for and inspect the in-line connectors from the firewall rearward, you'll likely find one of them filled with a nasty bit of corrosion that's killing the rear circuit.

 

No they don't

 

Then you know the problem is at a common point in the system upstream from the splice to the license plate lights.