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

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

$12 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.