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Help: Wiring a harness for a trailer

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For information's sake I am cross-posting from the Tesla Forums for anyone else that wants to tackle this job. Here's the reply that I got from a member who has installed a hitch and harness.
Hi
I'm not understanding how you isolate the left and right turn signals or the brake light . I have a wiring kit with snap on connectors . What do I do with the volt meter to locate the matching snap on attachments
 
Hi
I'm not understanding how you isolate the left and right turn signals or the brake light . I have a wiring kit with snap on connectors . What do I do with the volt meter to locate the matching snap on attachments
Paul, you may want to review the posts on the previous pages, which provide further background and details. The source of the problem is the fact that the Model S illuminates the tail light as a turn signal, as well as the brighter brake / turn signal light, which means that the tail lights on both sides blink when the turn signal is used when the running lights are off.

As detailed above, I used a low power 12 vdc relay to create a logical AND circuit with the tail light from one side switching the relay while the tail light circuit from the other side provides the power which is routed through to the tail light input on your isolator. In this way the tail light is not illuminated when only one of the Tesla tail lights is lit (i.e., when it is being used as a turn signal), but is lit when both of the tail lights are powered (i.e., when they are being used as tail lights, and not as turn signals).

You will need to run a wire from the second tail light on the other side of the to run to the relay (I mounted the isolator and relay on the passenger side and powered them from the panoramic sunroof circuit, through a separate fuse, and ran a wire with the tail light signal from the driver side through the rear frame to the relay).

It has been working perfectly now for a a number of years now.

I hope that helps.

Richard
 
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4 pages of posts and nobody says where they put the trailer plug. I just tapped into the signal lines with a normal splice straight to the trailer plug and it works without issues. No need for expensive contactless things.

Currently i just put the plug outside the car and close the trunk on it but obviously i should stop doing this. I was thinking of running it outside the car through the rear wall and down the inside of the bumper and out the hole for the hitch. But then the issue becomes WTF do i do with it mine its not hooked to the trailer? I could just shove it inside there but it would roll around and just be an all around bad idea, any thoughts on securing it somehow if i went that route? or any other places to mount the plug. Also I'm using a 7 pin connector so its not that small 4 pin one you're used to seeing.
 
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Hi Folks. Its been mentioned a few times in this 10 year old thread, that some have found 12V from the tailgate lift motor. Can someone please describe where to find it?
This for a Model S 2015 P85D, and needed for trailer wiring (and maybe other uses).

Thanx
 
Hi Folks. Its been mentioned a few times in this 10 year old thread, that some have found 12V from the tailgate lift motor. Can someone please describe where to find it?
This for a Model S 2015 P85D, and needed for trailer wiring (and maybe other uses).

Thanx
It has been years since I did it but it is on the passenger side in a cable bundle and is a larger wire compared to the rest. Sorry, but I don't remember the color but it should be obvious. Just remove the carpet/cover on that side, follow the wire down from the power strut and tap in where you find it convenient. Good luck.