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You will probably need a wiring diagram to chase this down. For some reason, the 3rd light is getting constant power. Doe this happen all the time or only when your headlights are on? I would try unplugging the brake light switch to see if it goes away.
It’s just one of those electrical things you need to trace through.
https://www.ericthecarguy.com/faq/solving-automotive-electrical-problems/electrical-issues
Take a look at the wiring in the trunk and see if it’s rubbed through anywhere. Most wiring failures are at points of movement and most circuits are switched on the ground side. So if you have a broken ground wire, the light would still be seeing power at all times and getting a ground from the break.
Other than visual inspection, you’ll have to check out BBB Industries or Autozone also has wiring diagrams online and start going through it.
It seems your car’s brake light switch has two, separate connectors — would that be because the 3rd brake light is separately wired from the other brake lights? If so that might be a clue to what’s going on — maybe check the output of the brake light switch if one of them is permanently on. Lacking any wiring diagram, check to see which wiring colors between the brake light switch and the 3rd brake light match to confirm which connector goes where.
Unplug the brake light switch down by the brake pedal and see if the light goes out. If it does, the switch is bad, or at least half bad. The brake light switch takes a +12v input from a fuse (orange wire) and when the pedal is pressed, the switch closes and sends the +12v out of two separate wires. One of the wires goes to the turn signal (white wire) where it lights the left and right tail light, and the other wire (light blue) goes to the 3rd light. So inside the brake light switch, one of the contacts (the one for the 3rd light) isn’t opening when the pedal is released, probably a broken spring.
It’s all the time
What’s all the time? The light or the switch signal? Was the light still on after unplugging the brake light switch connections?
Unless the wiring has been modified, the light would have to go out when the brake switch is unplugged because that is its positive source. The other side of the circuit is ground, so even if there were wiring damage the light would still go out when the switch is unplugged. Below is the wiring diagram.
Just to be clear, by “all the time”, you mean the 3rd light is lit even when the key is out of the ignition switch? If so, it points to a failure of your brake light switch, as others have said.
If the faulty 3rd light comes on only when the key is in the ignition switch and it’s turned to the Accessory or Run position, it points to some weird fault in a wire bundle where the wire to the 3rd light is receiving power and ground from another circuit that is powered through the ignition switch.
Hopefully, the brake light switch is your problem. If the problem is due to some previous wiring modification, like splicing in a trailer light circuit, a car alarm or an aftermarket sound system, you’ll be in for a frustrating diagnostic session.
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