Hey all, longtime lurker but first time poster here. First off, I want to thank all the long-time contributors to this site. The help I've found here has been instrumental over the years in figuring out numerous issues, from seats blowing fuses to HVAC issues and head gasket replacement. This time, however, I'm completely stumped.

The crux of the problem, this time, is that my brake lights are stuck on. Common enough problem, but on my '04, this only occurs when the headlamps are turned on. When they're off, the brake lights work just as they should. To be fair, when the headlamps are on and the brake pedal is pressed, the lights do get a tiny-teeny bit brighter. But not enough to be noticed by anyone driving behind me. I've went ahead and did the easy stuff: replaced brake light switch, checked for shorts in all visible wiring, spliced in two new tail light assembly connectors. Brake lights are still stuck on whenever headlamps are activated.

Any guesses? Thanks in advance for any help you can send my way!

 

Hmm my mind went the same place as yours: shorts in the wiring or connectors.

One question: is it just bright tail lights on the left and right that are affected, or does this behavior include the high center mounted brake light too?

 

@MassEditor I know this sounds silly did you check the bulbs and the brake light switch. The switch is easy to get at

 

Yes, the third brake light is on too. In fact, that's what first alerted me to the issue. I was letting a friend drive it and I was following. I thought he was riding the brakes the whole drive.

 

Thanks for the reply, Richard. Believe me, nothing sounds silly at this point! Bulbs are all brand new and I've replaced the brake light switch - twice, just to be sure.

 

@MassEditor That sounds like a wiring issue - and that will be no fun at all.

So it is like the circuit that controls the tail lights is mixed with the brake lights, so instead of just lighting up the tail element it is light up the brake light element as well.

Any chance you have an old trailer light hookup squirrel away somewhere, they can do that

 

So, my thinking here is that it must be short/headlight related, rather than brake light switch related. If it was the switch, the brake light would be on all the time (including when the truck is off / key removed).

I know HDC is supposed to trigger the brake lights, but the connection to headlights seems to rule that out.

You mentioned you spliced in new tail light assembly connectors. Just to confirm, have you checked the bulb sockets & the tail light assembly for rust/shorts? I think water gets in there pretty often. Don't have the workshop manual up at the moment, but I assume the brake light switch just provides a ground connection for the lights, so the lights would go on with a short anywhere along the circuit that provides a ground.

 

Maybe faulty headlight relays backfeeding the circuit?? Try pulling them, turning on the headlight switch and see if the brake lights stay on.

 

You guys have definitely given me something to think about. I've probably been thinking about this the wrong way - instead of a brake system issue, this is perhaps more related to the overall lighting system.

@Richard Gallant That's what I've been afraid of, a deeply buried wiring issue. I do have a four-plug trailer connector but I checked it and the wiring looks sound. I'll give it a closer look, however.

@boston4 I definitely think you're on to something there. Indeed, both taillights do let in water. But, bulb sockets and wiring in both assemblies look good. I believe you're right about the ground purpose of the switch. I will console RAVE again and try to trace down as much wiring as my patience will allow.

@ahab Another great suggestion, I'll give that a try this evening.

Thanks for the replies, this is definitely a great community here.

 

Does your 2004 model have the daytime run lights (DRL) module behind the driver's side headlights under the air filter box? Maybe that module or the resistor is going bad or corroded?
I don't even know if the brake lights are even part of the DRL system so I might be way off in left field on this matter.