How To Access Third Brake Light Wiring

Discussion in 'Gen 3 Prius Technical Discussion' started by Hybrid Hound, Apr 30, 2022.

  1. Hybrid Hound

    Hybrid Hound Junior Member

    Joined:
    Oct 5, 2017
    33
    15
    0
    Location:
    South West Florida
    Vehicle:
    2015 Prius
    Model:
    III
    I have really enjoyed reading all the interesting comments on the rear third brake light, but now the time has come to try and get the info I need. Does anyone know how to get to the wire(s) that feed the light? I will need to access it to energize my brake controller since crawling under the dash to tap into the brake light switch is extremely difficult. I read the very informative piece on replacing the wing on the hatch (well-written and the PDF with photos was exceptional) but just need to know how and where to tap into the power wire that feeds the high third brake light. Thanks in advance for your help. Oh, and if you know the color that would be great!
     
  2. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
    17,245
    11,724
    0
    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    It's light blue, whether you nab it at the center one in the hatch or at the rear combination light on either side.

    I would bet the combination lights are less trouble to get to for a trailer controller than the center light.
     
  3. Hybrid Hound

    Hybrid Hound Junior Member

    Joined:
    Oct 5, 2017
    33
    15
    0
    Location:
    South West Florida
    Vehicle:
    2015 Prius
    Model:
    III
    Thanks! That's a big help. I guess you mean by combination lights the module that plugs into the tail light socket? Will dig around and try to find it this afternoon. Not thinking my new trailer will give me any problems stopping, as it is only 200 pounds more than my last one. But, since it does have the brakes, may as well try and hook them up. I am questioning (and don't know how to figure it out) whether the re-gen feature will be reduced by using a slight amount of trailer braking. With no brakes hooked up, and careful driving (not over 60) I managed 37 MPG back home from a 130 mile trip to pick up the trailer. I am also in new territory since I intend to mount the brake controller (a Primus IQ proportional) in the back area on the left side as you look in, on the carpeted top of the little cubby. That way, I can tuck it all into the storage area when not needed. By using the signal from the third brake light, it will save a lot of wiring. I am using a four way flat that plugs into an adapter that converts to a 7 way standard trailer connector. I will be able to pull a small trailer with four way connector if I need to, just have to unplug the seven way and there is the four way ready to use. If this works, (or does not), to energize the trailer brakes, I will report back. Thanks again!
    BTW ... our "new" (to us) trailer is an Aliner Ranger 10. I have removed the propane tanks, the heavy marine/RV battery (replaced with my 6 year old Prius battery that I saved after installing the Lithium battery) , the water tank, the water pump, the gas stove, and the 3 way fridge are all out. Put in a little dorm 120 volt unit since we camp with hookups. When I am all done I will take it and have it weighed.
     
  4. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
    17,245
    11,724
    0
    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    For that location, I'd think you would want to just nab the blue brake wire at the left combination light, wouldn't you? It's inches away. The third brake light is considerably farther.

    The left and right combination lights are so called because they combine brake, tail, turn, reverse, and side marker functions.
     
  5. Hybrid Hound

    Hybrid Hound Junior Member

    Joined:
    Oct 5, 2017
    33
    15
    0
    Location:
    South West Florida
    Vehicle:
    2015 Prius
    Model:
    III
    We had a bodacious rain this afternoon, so will wait till tomorrow to hunt down the wire. That would indeed be the most logical place to nab the brake signal. It would be like 10 to 12 inches away from the controller. I have previously (like several years ago) installed the Curt plug and play trailer wiring harness, so I think I can find the blue wire in that bundle somewhere. I thought I had it all figured out initially by using the "brake" wire on the four way, but when you put your blinker on, the brake light gets power too. Not sure how all that works since it is a separate light in the back. The Curt connector makes the system work and I am not sure what is going on in that little magic black box but it seems to function correctly. At any rate, the hair thin wires kind of scare me but I will try and find the right one and connect it to give the signal to the blue wire on the controller harness. From there (on the other side of the controller to the seven way plug in) ) I will use 12 gauge wire like the Tekonsha people recommend. At least...that's the plan...
     
  6. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
    17,245
    11,724
    0
    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    Here's an old post where I described a way I sometimes deal with that, so I'm not molesting any hair-thin Toyota wires, and have the option to go back to stock any time and leave no sign I was there.
     
  7. Hybrid Hound

    Hybrid Hound Junior Member

    Joined:
    Oct 5, 2017
    33
    15
    0
    Location:
    South West Florida
    Vehicle:
    2015 Prius
    Model:
    III
    WOW. Very informative. A wee bit beyond my abilities and comprehension, though. I had to deal with tiny wires in a close, confined space a few years ago putting a four way flat trailer connector into a Chevy Volt. E-Trailer had the hitch, but no wiring kit that plugged into the tail light harness. I ended up splicing neuron-thin wires. Not brain surgery, but close LOL! I will first try and find the wire I need, then see if I can pull enough out to make a connection. All I need is a 12 volt signal when the brakes are applied. Sure hope this wire has enough amp power and is able to trigger the brake controller. We'll see! Thanks again for your input!
     
  8. Hybrid Hound

    Hybrid Hound Junior Member

    Joined:
    Oct 5, 2017
    33
    15
    0
    Location:
    South West Florida
    Vehicle:
    2015 Prius
    Model:
    III
    Thanks Again, ChapmanF. Since I knew what color to look for, I looked again in the left side opening by the lights after pulling out the little cover. There was the connector I used when hooking up the trailer lights, and the joining wires between those two points in the modules from Curt (that plug into your existing light modules) were MUCH thicker than the neuron size Prius wires. After disconnecting the two, and then putting my tester on the metal tab for the blue wire, the tester lit up just fine. It was showing 13 volts at that location when the brakes were applied. I cut the wire (about 18 gauge) and then spliced my wire in going to the brake controller. Got it all back together and now am getting a solid signal out the controller. Will be hooking it to the trailer soon. I put Velcro hook tape on the back of the controller and it stays put when placed on the carpet of the cubby. I will set it to a low level to begin with and step it up a little at a time until I get just a small amount of braking from the trailer. Knowing the color of the wire to splice into helped a lot!
     
Loading...