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TM3 Trailer Harness Wiring thread.

Sorn

Member
Jul 10, 2020
7
2
Los Angeles
Hey all. Im looking to actually use my torklift hitch with an actual trailer (and not just my bike rack!) so I thought I would struggle through the install. Now that I've spent a few hours researching all of the threads, I would like to condense the information here, ask some questions, and then update it once im done installing with pictures and helpful information. While there are a lot of threads on this topic, there isn't a really good "how to" nor is there a centralized thread with all of the relevant info. So hopefully this will be helpful to those after me!

Questions I have right now ....

-Is Fuse harness required? Top Amazon review says "THE KIT IS INCOMPLETE AND YOU WILL NEED TO PURCHASE AN ADDITIONAL "Wire kit 118150 or 118151 sold separately"

Is this true? I see another reviewer that said they installed it on their model S and it worked and a TM3 user who doesn't speak english very well complaining that it doesn't work. 118151 appears to be a kit to connect separate power modules? 118151 appears to have the fuse kit missing in the picture?
https://www.amazon.com/Tekonsha-118150-Trailer-Module-Wiring/dp/B01CKL9TEE

- Is there a reason people are paying $295 at the etrailer site for this same kit instead of $133 at amazon? going by the pictures I see there is a fuse harness box on the etrailer pic. Is this required to complete the installation? I read on the amazon description "Re-settable short circuit/overload protection - guards itself and the tow vehicle from shorts and wiring install mishaps; rated at 4.2 amps for turn/stop circuits, 7.5 amps for taillight circuit" ...so I was thinking if it has built in circuit protection the fuse harness is not required?

- What is the best route to run the 12V from the front to the rear?




I've ordered -

Tekonsha ZCI (no splice) harness wiring module
https://www.amazon.com/gp/product/B00HHRYPFK/

https://www.etrailer.com/Wiring/Tekonsha/119250KIT.html

There is a pretty good installation video from etrailer with this ZCI module that goes through which wires are your turn signal vs running light signal plus the GND location here -

TM3 Trailer Wiring Harness Installation - 2018 Tesla Model 3 - YouTube

Here is the easiest 12V power source -


And a link to the 5mm spade connector used to plug into that power source -

https://www.amazon.com/uxcell-1-187-Insulated-Female-Connectors/dp/B006Z95W8U/


For alternative power sources (but much harder to access) we have the rear trunk -

12v constant in rear trunk

To the left of the steering wheel there is the power supply that leads to the 12V cigarette adapter in the center dash (which seems a bit easier than accessing the cigarette adapter directly?)

Accessing the center dash cigarette adapter 12V -


Dome light 12V -

 
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Dutch24

Member
Jul 30, 2018
54
59
Guelph, Ontario, Canada
Hi Sorn,
I successfully installed a hitch wire harness in my model 3.
I picked up the curt brand harness (59236) from U-haul for $50 (cheapest one I could find).
upload_2020-7-22_13-58-31.png

I also picked up 'T' type connectors off amazon to prevent having to cut/strip factory wiring.
upload_2020-7-22_13-57-36.png

I ended up bringing the power wire all the way to the battery (some have gotten power source from under the rear seat, but read that some had issues with this as well).
I found that routing the wire through the interior up the passenger side was quite easy to conceal (I just pushed it under and behind the plastic pieces all the way to the trunk).
To get through the firewall of the vehicle you will notice that there is a fairly large rubber grommet with 2 red wires located on the passenger side under the dash (just remove the lower cover under the passenger side dash).
I used an ice pic and created a small hole and pulled the wire through.
Once through then I attached directly to the battery post, the curt kit comes with an inline fuse.
upload_2020-7-22_14-4-44.png

upload_2020-7-22_14-6-24.png

I used this video as reference only (different wiring harness), but it was a good source to determine wiring of the rear tail lights.
 
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Sorn

Member
Jul 10, 2020
7
2
Los Angeles
Hi Sorn,
I successfully installed a hitch wire harness in my model 3.
I picked up the curt brand harness (59236) from U-haul for $50 (cheapest one I could find).

I also picked up 'T' type connectors off amazon to prevent having to cut/strip factory wiring.
/QUOTE]

Thanks Dutch! Could you provide the link to the T connector you got on amazon? Im searching and can't seem to find the one you've provided a picture for!
 

Sorn

Member
Jul 10, 2020
7
2
Los Angeles

Awesome! Curious...is there any reason why I just couldn't wire it directly to a basic adapter like this one? As long as im using T connectors and not splicing into the rear brake lights there shouldn't be any issues ?

36” inch 5-pin Truck Pickup Trailer Cable Extention Adpater Light Wiring Harness | eBay

Or for a non-china option....maybe just cut off the male end of the adapter and splice right into left/right signal vs running light signal?

4-Pin Plug Trailer Light Wiring Harness Extension Flat Wire Connector 2ft 18awg | eBay
 

Dutch24

Member
Jul 30, 2018
54
59
Guelph, Ontario, Canada
Awesome! Curious...is there any reason why I just couldn't wire it directly to a basic adapter like this one? As long as im using T connectors and not splicing into the rear brake lights there shouldn't be any issues ?

36” inch 5-pin Truck Pickup Trailer Cable Extention Adpater Light Wiring Harness | eBay

Or for a non-china option....maybe just cut off the male end of the adapter and splice right into left/right signal vs running light signal?

4-Pin Plug Trailer Light Wiring Harness Extension Flat Wire Connector 2ft 18awg | eBay

I am not 100% sure. I did read someone has done that and was successful, but I believe this depends on what type of lights are on the trailer you are towing (believe again they must be LED lights on the trailer for this to work). Someone else explained that doing this would cause the lights to flash constantly and not stay on solid (so basically it would look like you had your fore ways on the entire time), but again not sure about this either.

Those cables are only 10ish bucks, you could give it a try and see. If it doesn't work you can always re-use the 'T' connectors and just connect the module piece then. Plus if you had to switch it out the curt module is only $50 compared to the $133 that you were going to maybe try anyways.
 
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Sorn

Member
Jul 10, 2020
7
2
Los Angeles
Ok, all finished. Actually managed to have everything working first try with the ZCI. Im attaching pictures of the 12V wire I ran to the blue wire that powers the center dash cigarette adapter because the "easy" 12v (first video in my first post) doesn't exist on my 2018 TM3. I took off the floor panel by the brake, got up in there...but that white box to plug in a 12v with a 5mm spade adapter simply didn't exist. It was easy enough to tap into the blue wire however, and made for a very clean route to the trunk. I used this video -


I would say it looks a lot easier to run the wire on the driver side vs passenger side because driver side rear you don't have to go through a firewall to get to the trunk and the wire needs to be on the left side as thats where the easy ground is anyways. You just have to pull back the panel to the right of the rear right seat and then you can run the wire along the other bundle of wires already going to the trunk with no need for modifications.

Simply zip tie your 12v going along the bottom of drivers side following the other cables and you should be good.

My biggest complaint with the one I purchased off amazon is that the cables are quite literally too short to reach from the left side to the right side. The white GND wire was the most limiting factor, though even when I extended it the ZCI wires running to the right signal (or was it stop?) was too short. It was so short that the ZCI was literally hanging in the air in order to connect both the left and the right side. How they could have made such a gigantic oversight is beyond me.

So what I did to fix it was basically removed the tubing/electrical tape along the TM3's brake light/signal wiring bundle so that I could pull the ZCI stop clip really far down the wire. Basically had to pull that little box almost 12" down from the TM3's brake plug so that I could tuck the ZCI's wires underneath the cover.

Also, the amazon kit didn't come with a fuse housing, which was easily resolved with a $1.50 purchase from my local hardware store. Im pretty sure every hardware store carries those automotive fuse housings so it shouldn't be a bother for anyone else to find one.

Once it was installed I just popped in the fuse, did Left / Right / Brake / Headlights for 5 seconds each and then when I plugged it into the trailer it worked first try.....rare indeed.

I think im just going to leave the ZCI installed and not bother with the cheaper one. Im just happy it works. I was worried the actual hitch adapter wasn't going to be long enough but it was by about 6" once attached to the trailer. So they at least didn't skimp on that one.
 

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smatthew

Active Member
Jun 9, 2018
1,254
2,051
CA Bay Area
I'm glad you decided to use the control module. Wiring directly to the lights was OK when cars weren't computers with wheels. These days it's a good way to make the car's computer go crazy when it sees unexpected power draw.
 

Dutch24

Member
Jul 30, 2018
54
59
Guelph, Ontario, Canada
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Threader

Member
Apr 16, 2019
500
306
Montreal
Here is a digital wiring adapter. It plugs into the Tesla trailer connector, so you don’t have to pull any other wires:

"Tesla Model 3 Trailer Digital Wiring Module (Trailer Hitch Lights Connector)"
Tesla Model 3 Trailer Digital Wiring Module (Trailer Hitch Lights Connector) | eBay

It's very pricey. It would be nice to know when using this if plugging in trailer lights does the Model 3 screen recognize / show that it knows there is a trailer connected? the ebay listing does not provide this info.
 

RedMistress

Member
Mar 15, 2020
10
4
London, Ontario, Canada
Most of my hitch use is for a bike rack but now and then towing the jet ski or utility trailer is desirable. I didn’t want to connect anything directly to the car wiring so I glued 2 photoresistors into small clear suction cups. Stick them on the car tail lights and drive 12 volt relays to switch the trailer lights. I have a long 12v lighter plug and run the cable between the seats to the console. Works perfectly and cost less than $30.
 
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grmdl3

Member
Apr 21, 2020
154
104
Oregon
Resurrecting this old thread. I just installed an EcoHitch Stealth on my 2020 Model 3. There is a plug on the harness under the bumper cover that sure looks like a trailer wiring plug. It's not plugged into anything, and IIRC has 4 wires on it that look to be large enough to carry power.

Is it still recommended to get something like this to isolate the trailer from the vehicle better?
 

ai4px

Wes
May 2, 2018
447
478
Sumter SC USA
Resurrecting this old thread. I just installed an EcoHitch Stealth on my 2020 Model 3. There is a plug on the harness under the bumper cover that sure looks like a trailer wiring plug. It's not plugged into anything, and IIRC has 4 wires on it that look to be large enough to carry power.

Is it still recommended to get something like this to isolate the trailer from the vehicle better?
There is a trailer light converter on ebay that connects to that gray plug... but it's $400... too high for my taste. I just used the Tekonsha 119191 kit. Worked like a champ. You use all brake wires from the tekonsha unit...


Passenger side: Red = right turn (green tekonsha wire),

Driver side: Purple = left turn (yellow tekonsha wire), Pink = lights (brown tekonsha), Grey = brakes (red tekonsha)
 

Pjschae

Member
Dec 30, 2018
33
6
Salt Lake City
I have the Tekonsha ZCI kit installed on my 2019 M3. It was working perfectly but now it is activating the brake lights when plugged in. I’ve disconnected it from power (VC left) and tried the reprogram sequence several times with no luck. Anyone else have this issue and a solution?
 

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