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Brake and reverse lights come on together. No tail lights

On my 2002 Dodge Dakota Sport, the brake lights and reverse lights always come on together if I am in reverse or applying the brakes. The tail lights will not come on at all. It has a towing harness, and the same symptoms are seen at the trailer light output. No tail lights, but the brake and reverse lights come on if I am in reverse or applying the brakes. I do not know how long this has been the case. I have checked the fuse box and inspected for disconnected grounds.

Without a wiring schematic it’s a hit and miss challenge to find the cause but I suggest you recheck the ground connection and inspect the bulbs for proper application and correct installation.

Odds are that you have a short to ground in the wiring harness, Generally these occur where the harness goes through a body hole. Whereas it’s had a trailer kit installed, that would be a good segment of circuitry to start looking. Sometimes it’s easier to run new wires than to find the short in the old ones.

You can generally find exactly which wire it is with a few long lengths of 16ga stranded wire, a few alligator clips, and a multimeter. Or just run a new harness.

Thank you for the quick responses! I will begin delving deeper with the multimeter. I checked the sockets, and they are definitely receiving voltage on the same wire. All the grounds have continuity to the frame at the sockets. Time to move upstream.

Factory tow wiring, Mopar tow wiring or an aftermarket splice job?

Switch on the turn signal and check if the reverse lights flash.

BTW a short to ground will blow a fuse.

I believe it’s a factory kit. The turn signals function normally and do not trigger the reverse lights.

The reverse and brake lights should not have the same wire. The wire for the reverse lights should not be shared with anything. Depending on the vehicle, the rear turn signals and brake lights, but not the 3rd brake light may share wires.

The brake and turn signal share the same wire and bulb for each side so this doesn’t make sense. All the rear lights share the same ground so if the rear ground was lost the brake light could find ground though the parking light circuit but the reverse light would not light up.

The factory supplied kits were referred to as “Mopar accessory” kits, these wiring kits use inline connectors near the tail lights, no cutting and splicing. If this is a Mopar kit there should be a Mopar sticker with tow limits listed on the hitch. Is there a brand name on the hitch?

I am going to guess that

  1. When the reverse and brake lights illuminate, they are not as bright as they should be because they are functionally wired in series…
  2. You can prove this theory if you remove the brake light bulbs or the backup bulbs, the other bulbs will not light.
  1. Both lights illuminate when you activate one set but not when you activate the other. (e.g when you press on the brake but not when you put the car in reverse).

If I am correct, the ground for the brake and backup lights is an open circuit (disconnected). When your continuity tester shows a ground connection, you are actually measuring through the bulbs and finding a ground in the relay that controls the bulbs That relay grounds the hot wire when the circuit is turned off.

If I am correct. find the ground trace in your tail light and make a new connection between that trace and chassis ground. Doing one side will probably fix it but best to do both sides.

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That does sound like the problem @Manolito. Feedback through and ungrounded ground wire can cause some odd problems.

Both the backup and breathe lights come on when I activate either, but the tail lights never come on. When I unplug the light sockets to test, each ground had continuity, and the lights were out of the circuit. There is 6v at the same wire whether I apply brakes or reverse. No voltage at any wire with headlights I am testing first at the socket plug, then upstream at the inline connectors for the trailer harness.

getting 6 volts is indicative of feedback through a working bulb.

I will try to further isolate each light before taking further measurements.

When I’ve had similar problems like that on my truck, it is usually caused by the unused trailer connector shorting out against something on the truck’s body. If so, wrap the connector inside a plastic bag or something. If that’s not the problem, suggest to temporarily remove the trailer harness part of the wiring, return the wiring to the way the truck came from the factory, see if that makes the problem go away. If so, at least you know where to look for the source of the wiring problem.

For a '97 Dakota, the ground for all those rear lights is described as “near the battery” so you might check there. Also, you could try replacing the brake light bulbs in case a wacky filament failure has occurred.

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Similar problems have shown up at my door on Fords and Jeeps and as stated up there somewhere a failing ground and/or bulb installation has proven to be the problem. But often the trailer wiring was involved with all manner of tail/brake/back up light issues.

I had a short tracer device that blinked a current down whatever circuit it was connected to and that vastly reduced the time spent chasing problems since it eliminated shorting fuses and keeping someone else busy operating switches. There was a compass with the kit that would sweep with each pulse of the flasher to keep track of the circuits involved.

And here I’ll add that drilling a hole in a hidden area of the body and adding a redundant ground will do no harm and may eliminate the problem.

I know this is of no help, but I have to ask. When did manufacturers start including a reverse light lead in trailer harnesses? I have never seen this or a trailer that even had reverse lights. Or is this some specialty trailer and custom harness?

I don’t think it is a bulb issue, as the voltage shows the same behavior regardless of whether the bulb sockets are in the system or not. I will certainly try removing the trailer harness from the system to see if behavior changes. There is a canopy with a brake light, but I have isolated that from the system with no change as well.
As far as the trailer socket, it is a 7-pin mounted socket, like this one:image
I am not sure when they began incorporating a backup light, but according to the internet the center pin is for a backup light.
I use a 7 to 4 pin converter, so I should never have backup light voltage through my harness. The 4-pin converter shows the same behavior (brake lights come on in reverse and when braking.

Many years ago I posted a puzzler regarding strange behavior of tail lamps etc on a friend’s car. We found the ground wire in the trunk had been knocked off during loading stuff into the trunk. I have a few vehicles that have similar grounding techniques with a spade terminal and a single black wire providing the rear ground to chassis. The lack of a dedicated ground and the lamps providing the circuitous path led to some strange behaviors…