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Old 08-07-2019, 04:00 PM
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Wiring 101 - tail light wiring - question

I have re-wired two JK's so that I could run the license plate and integrated third brake light (rather than the factory) without issue. Today I wanted to finalize the installation of a rear bumper with tire carrier by installing an integrated third brake light and license plate light.

I have the license plate bracket all wired up, with both ground wires for the brake and license plate lights tied together and then I have a second and third wire, one each for the brake and license plate. I joined them using a three wire connector (weatherproof) so that the bracket can be removed if necessary to get to the spare. I tested my wiring to the license plate/brake light bracket using a 12 volt supply and with a common ground, each remaining wire lights up one of the lights, so far so good.

I then ran the three wire cabling from the mounted spare in a nylon sheath across the tire carrier and over to the passenger side brake light. I pulled the wiring bundle in its sheath from under the body to the hole for the wiring for the passenger side rear tail light. I unscrewed the tail light from the body and found three wires along with the light being grounded by the body. I then bared a portion of each of the three wires on the factory light wire harness and go about testing to see which one is brake, turn signal, reverse, and tail light. Regardless of what I do, I cannot get my add-on lights to light up when using any of the three factory wires. (yes, I have everything grounded). What am I missing? I need the license plate light to illuminate with the tail lights, and obviously the third brake light to light up with the brake lights.

I'm about to dig into the electrical schematics in the Service Manual, but thought I ask the wisdom of the group too.

Also, I noticed that the drivers side tail light has a connector to unplug from the wiring harness, but the passenger side appears to be hard wired, unless the connector is hidden somewhere up in the rear fender.


PS. I also wired in a backup camera today (inside the trailer hitch receiver) and needed to connect a wire on the drivers side to the backup (reverse) light. I think I did it correctly, but pending understanding of the tail light wiring setup, I guess I'm not certain.

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Old 08-07-2019, 05:14 PM   #2
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I power anything not OEM thru Bosch relays. Minimizing drain on the crappy OEM wiring.

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Old 08-07-2019, 05:24 PM   #3
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Here is how to wire a Bosch Relay.
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Old 08-07-2019, 05:38 PM   #4
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Which jeep? Your TJ, JL or a JK mentioned above?
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Old 08-07-2019, 07:56 PM
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I too believe in relays, in this case, I wanted the lights to work off the exiting switches, head lights, parking lights, brake lights. Don't understand how I'd get a relay to do that.

As for my handiwork, I scored a 2 1/2 out of 3...... on the TJ. Meaning I had a failure along the way.

I did find the reverse light to power the switch that turns on the backup camera. A+

I did find the wire that will light up the license plate when the headlights/parking lights are turned on. A+

I did find the wire that will light up the third brake light, when the brakes are applied. However, the third brake light also blinks when the turn signals are turned on!! A+ / F-

What in the world am I doing wrong on this TJ. How do I get just the action of applying the brakes to light up this third brake light?
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Old 08-07-2019, 08:21 PM   #6
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You have to tap off the center brake light (white/tan wire). You can access it either in the tailgate or in the wiring harness under the tub crossing from driver’s side & going up to one of the 2 buttons in the tailgate opening on the right side. I think it goes up into the rear wheel well to attach to the button.

The reason you can’t use either of the taillights is because they use a single bulb filament in the taillight assembly for both signals & brakes. They basically blink the brake lights as signals. That means the l & r brake light circuit goes through the multifunction switch which turns the brake light off & switches that wire to the signal (through the flasher) for the desired side (or both for the hazards). The center wire goes directly from brake switch to center light.
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Old 08-07-2019, 08:41 PM
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Quote:
Originally Posted by RUBI 4 MY MRS View Post
You have to tap off the center brake light (white/tan wire). You can access it either in the tailgate or in the wiring harness under the tub crossing from driverís side & going up to one of the 2 buttons in the tailgate opening on the right side. I think it goes up into the rear wheel well to attach to the button.

The reason you canít use either of the taillights is because they use a single bulb filament in the taillight assembly for both signals & brakes. They basically blink the brake lights as signals. That means the l & r brake light circuit goes through the multifunction switch which turns the brake light off & switches that wire to the signal (through the flasher) for the desired side (or both for the hazards). The center wire goes directly from brake switch to center light.

Ugh!
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Old 08-07-2019, 08:47 PM   #8
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If you already ran your new brake wire to the passenger side it should be pretty easy to move it to the proper wire now. Easier than your last project, right??
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Old 08-07-2019, 09:43 PM
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If you already ran your new brake wire to the passenger side it should be pretty easy to move it to the proper wire now. Easier than your last project, right??
Yes, much easier, but as I reflected upon the job I did today, I was a bit unsatisfied with the workmanship. I was hoping that there was an easier answer than fishing out another wire.

While considering that task, I've decided to rip it all out and start over, neater, nicer, better wires, more shrink wrap, and better solder joints.....

The Ugh was the realization that I basically need to start over......LOL

Tomorrow is my B Day, so its likely that the wife will protest me spending the day by myself in the garage, so the work will have to be pushed to Friday.
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Old 08-07-2019, 09:58 PM   #10
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Happy b-day
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Old 08-08-2019, 07:15 AM   #11
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Explain to the Mrs., that you need to work on fixing this critical safety issue. After you fix it, it will be safe enough to drive to the restaurant and she can buy your birthday dinner. I'm sure she would understand... Yeaah, that's the ticket!


Happy Birthday!!!
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Old 08-08-2019, 07:17 AM   #12
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Here is how to wire a Bosch Relay.
Simple, but one of the best diagrams to clearly explain how to set up a relay on an accessory circuit. - Thanks!
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Old 08-08-2019, 11:45 AM   #13
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I learned about Bosch Relays when I was learning about how systems are wired in transit buses.

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