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Discussion Starter #1
Recently picked up a new to me 2000 Passat for a project.
Everything has been going great as far as getting it road worthy, however today I noticed a sort of “interesting” snag.

Upon checking all the lights, we noticed that the brake lights work fine with the headlights off, however when you turn the headlights to the on position they no longer work. With the headlights on, the 3rd brake light will illumate when pressing the brake pedal, but there’s no change in the tail/brake light combo. However, with the headlights off, everything works fine and illuminates as it should.
I replaced both bulbs today, and there was no change in the behavior, and yes I made sure to use the correct dual filament bulbs.
Normally, I would go straight for the brake light switch, but with them working normally when the headlights are off, and the third brake light working regardless I don’t suspect this to be the issue.

Anyone have advice on what I should check next? It’s not safe to drive it at night as it is.
 

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Any rust on the car or water intrusion? If so, look for corrosion on the wiring for the grounds on the battery and frame. I am not sure where the taillights ground to, but I suspect under the cover behind the hood release. What are the battery voltages with the headlights on, off and the same with just the brake lights?
 

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I tend to agree with PZ--it really sounds like a faulty ground. (The lamps operate in series to find a way to ground, until you apply +12V to the parkling light circuit. This works because cold incandescent lamp filaments have very low resistance.)

That said, with a bad ground and headlights on, I would expect the tail and brake lights to go dark when you apply the brakes, so I'm not certain. It would be interesting to know how the turn signals/hazards behave if you apply the brakes...

The ground wires in our cars are brown. Per the Bentley schematics, the tail light assemblies ground in the rear of the car. It shows Right grounding on the right rear pillar, which I believe. But the left says "left rear window wiper", which makes no sense.

Regardless, the ground on the right, at least, is near where the Monsoon (premium audio) amp mounts, so a previous owner may have screwed around with that.
Start with checking the grounds. If you have a multimeter, unplug the connectors from the taillights, then check the resistance to ground of the brown wire in the harness.

Sorry for the ramble...
 

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Discussion Starter #5
I was thinking the same thing. The car is super clean, no rust of any kind.
I bought the car from a friend and he wasn’t good at maintaining it.
I will check all the grounds once I am home from work today.
It just stuck me as odd that they function perfect with the headlights off, but don’t work when they’re on.

All the blinkers and hazard lights behave as they should, regardless of the headlights being on or off.
It is specifically the brake/tail combo.
 

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So I took a look at the circuit layout, and am stumped on how this situation occurs. The brake lights are totally independent of the headlights, and that third brake light is in parallel with the two main brake light bulbs. The only thing that comes to mind is to measure the ground resistance between the tail light module's ground, which should be the stamped steel bulb holder, and body metal. If that ground resistance is high for some reason, turning the headlights on will also switch on the tail lights, increasing the Voltage drop. Mayby the 3rd light has a different path to ground, although the schematic doesn't show that.

Besides doing that, try removing one of the bulb on one side and see if the other now lights.
 

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Is it only the brake lights that will not illuminate? Do the tail lights work when you turn on the headlights?
I suggest taking a multimeter to the brake light wire on each assembly and testing there with headlights on and off and brake applied. What voltage does it read? Use your own ground point for the negative probe, then test again using the ground point/wire for the tail light assembly.
 

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Is it only the brake lights that will not illuminate? Do the tail lights work when you turn on the headlights?
I suggest taking a multimeter to the brake light wire on each assembly and testing there with headlights on and off and brake applied. What voltage does it read? Use your own ground point for the negative probe, then test again using the ground point/wire for the tail light assembly.
I concur. I would make a table three columns by three rows (or 4 x 4 if you count titles).
The columns are headed: ground, brake, tail
The rows are titled: brake only, tail only, brake & tail

With brake on, measure to the ground connection on the light bulb base (brown wire), the brake light connection (red/black), and the tail light connection (grey/red). Repeat with lights on, then repeat with brakes and lights.

So, you'll have 9 voltage measurements, all with respect to a body ground (i.e. negative lead to body). Post those measurements and we'll figure out what's happening.

BTW: this is both tail light assemblies, correct?
 

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Discussion Starter #10
Well guys, I figured it out.
Turns out the previous owner put both bulbs in backwards and I didn’t even realize it. Both bulbs have been flipped and worked as intended now!
 

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🤦 Ha! Sometimes, it's the simplest of things.
 

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I replaced both bulbs today, and there was no change in the behavior, and yes I made sure to use the correct dual filament bulbs.
Sooo, you put the new bulbs in wrong too? I considered mentioning incorrect bulb installation in my first post, because this is not an unheard-of problem.
 
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