Hoochapotamus

Title says it. After scouring the forums and internet in general, I was able to piece-meal together some good info and test it on my 2017 XLT. I bought a used Leer topper that came with a 3rd brake light and interior dome light. Both were wired to a mounted "TAG" (Truck Accessories Group) fuse box inside the topper (pic attached). Coming off the bottom of the fuse box were 3 cut wires, black, red, and red with white stripe (pic of not-cut harness attached). Found some pics of the instructions from TAG, indicating Black = Ground, Red = Dome, and Red/White = CHMSL. Since Ford uses one bulb for both brake/turn, it makes things a little tricky. There was a lot of mixed info from different sources regarding a pre-ran/capped wire for wiring a 3rd brake light. I looked near the trailer hitch wiring, and under the hood near the master brake cylinder and did not locate this on my year of truck. I found a screenshot of instructions from TAG indicating on 2015+, there is a purple/white stripe wire in the white/grey plug located behind the driver side kick panel. On my truck the 2 plugs located there, one black and one grey. I read on another thread that the purple/white stripe wire was in the top plug, with pics of their install. On mine, I located it in the bottom plug (who knows maybe they got flip flopped at the factory because they attach to the same type of mounting connector). To test, I used a multimeter with a piercing probe going into the purple/white stripe wire and the other probe grounded. Multimeter read 0.0 volts until I depressed the brake pedal, whereafter the multimeter jumped to ~12 volts, confirming this wire only receives power when the brake pedal is applied, aka the CHMSL wire. From there it was just a matter of connecting everything. Cut/solder/shrink wrapped a ~15 foot piece of wire to use as a pig tail to run back to the tail light. Pulling back the driver carpet slightly reveals a rubber grommet the wire can be passed through. Ran the wire through, slathered the grommet in some liquid tape, and then routed it back to the tail light. Inside the bed, near the tail light, there was a hole that goes into the back side open space behind the tail light. Removing the tail light presents two plugs, one with two wires and the other with three. The three wire plug is the brake/turn/taillight. Reverse light is the 2 wire plug. On the 3 wire plug, black is ground, middle wire is taillight, and other one is brake/turn (sorry I forgot to write down the 2 non-black colors). Cut the black wire and middle wire. Connect the black wire coming from topper to the black taillight wire. Connect the solid red wire coming from topper to the cut middle taillight wire. Connect the red/white stripe wire to the newly ran pigtail from the driver kick panel. Everything works as it should. Third brake light illuminates when brake pedal is pushed, and the interior dome light clicks on/off if the taillights are on. If you wanted to be able to use the topper dome light without the taillights on, I would recommend installing an add-a-circuit to either a keyed or constant powered circuit under the hood or in the passenger kick panel fuse box. The whole process took more time researching and testing than actually hooking things up. Hopefully this post will help someone else looking to connect or add a topper or 3rd brake light. I'd read of people connecting to the trailer harness, making the computer think a trailer is always connected, or wiring everything to the taillight and having the 3rd brake light on the topper blink when your turn signal is on.

TLDR & Meat and Potatoes:

2017 Taillight Wires: 3 wire plug = Brake/Turn/Taillight Black = Ground (-), Middle Wire = Taillight, 3rd wire = Brake/Turn
2 wire plug = Reverse light Black = Ground (-), Other Wire = Positive (+)
2017 Driver Kick Panel Plugs: One grey and one black plug. The CHMSL wire on my truck was in grey plug on bottom.
3rd Brake Light/CHMSL Wire = Purple/White Stripe (recommend testing with multi-meter and piercing probe prior to cutting any wire)
TAG Fuse Box: Plug/Harness coming out of the "To Chassis" side with 3 wires: Black = Ground, Red = Dome, Red/White Stripe = CHMSL

How to connect everything in taillight: 1) I recommend solder or waterproof butt splice and heat shrink as opposed to t-tap. 2) Cut and connect black wire from topper to black wire in taillight. Cut and connect middle wire on 3 wire plug to solid red wire from topper. 3) Connect newly ran CHMSL wire from driver kick panel to red/white stripe wire from topper.

Tools I used: Wire stripper, soldering iron and solder or waterproof butt splices, heat shrink tubing, ~20 feet of wire (I used 16 gauge speaker wire), split wire loom (Harbor Freight has it cheap), zip ties, tesa tape/friction tape, cable sleeve for inside truck, liquid tape, and a lighter.

Pics below show which connector I found the CHMSL wire in, where I routed through the floor, the tail light pulled, and the info sheets I found from TAG.

WVMoose

iTrader: (2)

Not sure how different a 2019 is from a 2017 but I had a lower and the same TAG connector.

my cap also had the 3rd brake light and interior LED light. Mine also had a lock that locked the cap with the key fob with the truck locks

I thought I’d be able to splice it in and got a lot of people warning me of electrical issues.

anyways, I gave up after hearing too many horror stories. Took it to a leer dealer, they charge me $100 for a harness and $100 to install. It piggy backed the 7 pin plug, ran power from the battery (fused), and spliced to a wire by the driver side door sill.

wish you luck!

Hoochapotamus

Yeah the TAG fuse boxes have the lock/unlock although I didn’t hook it up yet. I’m guessing in the fuse box under the hood there’s the door locks circuit which could be wired to the topper. Mine didn’t come with the plug/wires for the fuse box but I’ll probably scrounge up the harness and do that in the near future. Thanks for the input!

bheld

Have you had any issues with the truck thinking there was a trailer connected?

WVMoose

iTrader: (2)

nope

thats how they sold me on the harness, they said without it a lot of people got the false notifications

i don’t have the topper anymore so no real long term trials

Hoochapotamus

I haven’t had any issues either, neither with the truck thinking a trailer is connected (why I avoided splicing or making a t-harness, google EZWheeler 7-Way for reference, $120 but out of stock) nor with the third brake light flashing. I’d be more than happy to answer any questions. My main thing was I didn’t want to spend $100+ dollars on a harness or shop to connect a couple wires.

flyboy50sc

just a thought here. Since I am not really using the trucks 3rd brake light now as the cap blocks the view of it, if I unplugged the connector from the trucks 3rd brake light and had a connector from another one and just tapped the power for the topper 3rd brake lite and ran a wire to the back, that should not create an issue right?

Hoochapotamus

Yeah that would totally work. Not sure what kind of connecter the factory 3rd brake light uses, but you could definitely find a male or female connector that matches, pin it yourself with ~20 ft of wire and then run it out of the cab to the back. Could also use t-taps on the factory 3rd brake light wires since the connections would be inside, or cut them and solder ~20ft pigtails on. The only caveat is that you’d be running wire in the headliner down to somewhere on the floor to get the wire out, and at that rate tapping the chmsl wire might just be easier. Long story short yes that would totally work.

flyboy50sc

I found the wire yesterday (purple/white stripe) under the drivers kick panel, and my concern is the length of the wire available to work with and the small gauge of it. Ford did not leave very much there to work with. It would be very tight quarters trying to do anything up under there to do any splicing/soldering. Its gotta be easier trying to run wire back down along that back window and thru the seal on the back of the cab. Just my 2 cents

And now that I think about it, hook up some dome lights back in the cap to the old bed lights on the 3rd brake light, and have switched dome lights from the cab

Hoochapotamus

Yeah it’s definitely tight quarters down there. I unplugged the connector with the cmhsl wire and slid it out of the mounting connector thing, unwrapped the tape around the wires and cut it, ended up with probably 5” of wire to work with. Soldered and glue heat shrinked it, wrapped everything back up with tesa tape and was good to go. If you’re not comfortable soldering butt splices and glue heat shrink for peace of mind would work down there, or up near the 3rd brake light.