4-pin trailer to third brake light for Softopper

After purchasing a Softopper for my 2016 DCSB, I knew I wanted to do the right thing and install a third brake light on it. One of my primary requirements was to not mess with the factory wiring harness. I believe @Puppypunter has the most elegant (and affordable) solution in his tail light harness adapter, but me being impatient came up with an alternative while I wait for that harness to ship. My idea was to use either the 4 or 7 pin trailer hookup and use parts that were easily found at Lowes or Amazon. I figured I’d share how I made that solution work with anyone who may be interested.

Parts:
- DOT approved LED trailer light

$13 TRUE MODS 16" 11 Red 12V LED Trailer Light Bar [DOT Compliant] [IP65 Waterproof] [Park Brake Turn Signal] Running Marker ID Rear Tail Lightbar for 80" Cargo Motorcycle Utility Marine Boat Trailers

https://www.amazon.com/dp/B07646BHVC
- 4-pin extender

$10 WOFTD 4-Way Trailer Wiring Harness with 5 ft Wires 4-Pin Trailer Wiring Jacketed Cable 4 Flat Trailer Wire Extension Connector - 5 ft

https://www.amazon.com/dp/B08BNBZR4T
- 12-volt relay

$12 EPAuto 30/40 AMP Relay Harness Spdt 12V Bosch Style - 4 Pack

https://www.amazon.com/dp/B072QXDZRD
- Weather proof single gang box
https://www.lowes.com/pd/TayMac-1-Gang-Gray-PVC-Weatherproof-New-Work-Old-Work-- --Standard-Switch-Outlet-Wall-Electrical-Box/4005489
- Weather proof box cover
https://www.lowes.com/pd/TayMac-1-Gang-Rectangle-Plastic-Weatherproof-Electrical-Box-- Cover/5004945479
- 2 water tight conduit fittings
https://www.lowes.com/pd/Sigma-Electric-ProConnex-1-2-in-Uf-Cable-Connector-Conduit-Fitting/1087617
- An 8 ft extension cord
- Some wire nuts
- Heavy Duty Velcro
https://www.lowes.com/pd/Scotch-2-Pack-1-in-Black-Roll-Fastener/999989998

Note: I do recognize that this solution ended up being more expensive than waiting for a harness from @Puppypunter

Instructions:

Attach the conduit fittings to the box.

Snip the female end from an extension cord and strip the wires. I used an extension cord because the light I purchased had 3 wires. The extension cord serves as convenient disconnect if I want to fold down the Softopper.

Save the male end of the extension cord as it will be wired to the light


Figure out where you want to run your 4 pin extender. I ran mine behind the bumper and up into the bed cubby.


Feed the wires into your electrical box and tighten down the conduit fittings. I used a little dish soap to help them slide through.


Wire up the relay. Why a relay? Well a 4-pin trailer connector only has Left (brake/turn), Right (brake/turn), Tail lights, and Ground. If you just tap one of the Left or Right wires, your third brake light will blink. The relay is used like a logical AND gate to ensure the third brake only turns on when BOTH Left and Right brake/turn wires have signal. This means it will also flash when hazards are on. Here’s how I wired up the relay and extension cord tail:

4-pin Green -> Relay Blue (positive)
4-pin Yellow -> Relay White (switch)
4-pin White -> Relay Black, Extension Cord Green (combined grounds)
4-pin Brown -> Extension Cord Black (tail light signal) - This is optional if your light only has 2 wires
Relay Yellow -> Extension Cord White (Accessory) - This is the ANDed L/R signal which activates the third brake light

Shove all the wires and relay into the box and put the lid on


Finally, wire up the third brake light to the male end of the extension cord:
Extension Cord Green -> Light White (ground)
Extension Cord White -> Light Red (stop/turn)
Extension Cord Black -> Light Black (tail/parking)

I ended up soldering and heat shrink tubing all of this, but this picture makes it pretty clear


Plug it in and test it out


Cut a small notch in your cubby cover. I drilled a hole near the edge and used some side cutters to reach the edge


To attach the light to the Softopper, I put some velcro on both the topper and the light. This is the plastic water proof velcro, not the fuzzy kind. I ran the wire up under the flap and overall I think it ended up looking pretty clean!


Again I recognize this is not nearly as straight forward as tapping your tail light, but I hope those who do not want to touch their factory harness find it useful.

 

Perhaps not THE most elegant solution, but definitely well thought out and executed. Good work!

 

Awesome! The use of the relay solves the problem of the light blinking when the signal is activated!

Thankyou for the write up.

 

Can you verify that this does work. I duplicated it based on your notes and I got this.

 

i would advise against running the marker light circuit to the CHMSL location. opinions vary on the subject, but DOT regulations state the CHMSL to only be active for stoplamp duties. the primary arguement is that dimly illuminating the CHMSL can be argued to cause contradictory signaling to other motorists.

otherwise, your relay setup is correct. the only time the CHMSL will activate via the relay is if the brake or hazard lights are engaged.

 

Thank you for that. I am not using this on my truck, I'm installing it on the back of my 4x8 trailer. But I do agree so I will not be connecting it as a marker light. Instead I will connect both wires together on the third light, connect them to the brake output from the relay, and eliminate the tail light feature.
Cheers

 

Updated Schematic.

 

typically the marker light circuit would be cut off/capped at the light. for the majority of lights, the marker input is just a higher resistance that lights up the same led's, only dimmer. so connecting both the brake and marker, the brake signal already overrides the marker light.


that said, i do know of at least one combo S/T/T/id-bar that light up separate led's for marker/id and stop functions, at the same brightness, which would benefit from the wiring diagram as you posted it. but it's not a typical method for the majority of products on the market.

 

Agreed. This one has two separate sets of LED's

 

I have a concern.
I have laid out the possible outcomes. Can you verify that I am correct?

There are just 5 possible outputs
---------------------------------
0-Left & Right Off (default)
1-Left Blinking & Right Off (left turn)
2-Right Blinking & Left Off (right turn)
3-Both Left & Right Blinking (hazard light request)
4-Left & Right Steady (pedal pressed)
---------------------------------
The relay will only engage under conditions 3 or 4
If Left or Right is blinking & Brake pedal is pressed than both Left and Right will go steady. Brake pedal request over-rides turn signal requests thus changes the blinking light to a steady light.

 

This is what I thought but from what I have found the brake light does not override the turn signal so how could this work correctly? As it is the third brake light would flash if the pedal was pressed and a turn signal was activated.

 

yep, that's correct.

i've been staring at a few relay diagrams trying to figure out a mechanical logic method to this problem and haven't come up with a great solution. i know it can be done, but the circuit layout is escaping me. adding a delay would filter out the signal, but also erroneously come on when the turn signal is applied. additional relay's could work, but would require at least 2 diodes, and constant power, but would also flash with only one turn signal activated....

so i googled.

turns out $16 is far cheaper than the stress all this thinking puts on my brain.

https://www.etrailer.com/Accessories-and-Parts/Pacer-Performance/PP20-702.html

i did some more digging, because my curiousity is dangerous. and came up with this
https://67-72chevytrucks.com/vboard/showthread.php?p=4573566

this is the relay-only method. requires 3 diodes, 2 standard SPST relays, and 1 DPST relay:


and there's also the transistorized version as well


and this is the same circuit, but with more description
https://mgaguru.com/mgtech//electric/et240b.htm

 

Ahh, these are promising. I will study these. A Thyristor!? wow.
Thank you.

My Tundra has a built in brake controller. I could use the blue wire but I'd rather make a circuit to share.

 

and if you read through the chevy truck link, they also suggest that it'd be possible to use all SPST relays, and just use two in the place of the DPST relay. that method would offer easier access to automotive-grade/style relay's, but the bulk of the additional relay could possibly introduce some other packaging concerns depending on the installation.

 

I missed that. Read it now and found a component version by another author. I'll pass it onto a group I belong to to figure out what the components are as the photo is a bit blurry.

 

I talked this over with the group and this version works just like the 3 relay version. Still having the .5 second delay under certain conditions. I'm trying to eliminate the delay. We will keep plugging away at it.

 

This is what the group has come up with. I will order the components I dont have and test it.

 

if you're interested in offering/producing this for others, i'd suggest adding a notation on the board for the max allowable current limit of the added third brake light. most SMD components are only going to allow about 0.5-1A, and plenty of people go nuts with lights larger than that.

 

The diodes (D3-D4) are rated at 3 amps and the mosfet can handle it but I wont be producing this.