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Discussion Starter #1
I found the connector, thanks to everyone that posted before me.

I built the wiring harness with relay. Before I connected anything I thought I would test what I was connecting to.

Should I be getting 12vdc from the the hidden brake controller connector? I have the Touareg running and I am not getting any voltage across any of the pins.
 

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That's the one....
Thanks
No voltage. See post #6 here: Factory Towing Option and Brake Controller Problem - AudiWorld Forums

Relevant parts of that thread quoted below in case the link dies or is blocked.

DUTCH VanAtlanta @ Audi World said:
Today I was going to install the Tekonsha P3 controller per the instructions in the attached bulletin, but ran into a possible glitch. In checking the pins in the under dash connector, I found that pin 3 in the connector does not get 12 volts as it should when the brake pedal is pressed; and without it getting that brake pedal 12 volt signal, the trailer brakes will not get any control signal through pin 4. I do have the ignition on and the engine running with the Towing Mode selected in the MMI. The other pins in the connector are as they should be. Pin 1 is ground and pin 2 has always on 12 volts.
DUTCH VanAtlanta @ Audi World said:
2.) Dealer's Service Manager, while not sure of the reasons for no power to pin 3 of the under dash brake control connector, had me come over and plug in to a boat trailer they had just taken in trade.

Plugging in to the trailer electrical socket next to the hitch put enough load on the system to activate the Towing Recognition Control Module in the Audi factory setup. This gave proper operation of all light functions on the trailer and 12 volt power to pin 3 in the under dash connector when the brake pedal was depressed.

It really is a simple plug and play; but you have to have a non LED load on the circuits to get them to operate properly.
 

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Depending on the model year, your Treg may recognize an all LED trailer. I know mine does just fine. You can also buy a tester to plug into the trailer socket.
 

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Discussion Starter #6
Depending on the model year, your Treg may recognize an all LED trailer. I know mine does just fine. You can also buy a tester to plug into the trailer socket.
Thanks... Very helpful.. It sounds like I should still be getting voltage between pin 1 and 2 without the trailer connected?

This is a 2016 TDI.

I will get a 7 pin trailer light tester and see if that turns on the trailer functions. Maybe a blown fuse as well.
 

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Discussion Starter #7
It was fuse 5 on the passenger side that was blown and not providing a constant 12vdc.

The 7 pin trailer light tester did activate towing mode even with the small LEDs.

I will pick up the trailer on Tuesday.
Thanks...
 

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Thanks... Very helpful.. It sounds like I should still be getting voltage between pin 1 and 2 without the trailer connected?

This is a 2016 TDI.

I will get a 7 pin trailer light tester and see if that turns on the trailer functions. Maybe a blown fuse as well.
Not quite. I believe that starting with the 2013 model year, the trailer modules were redone or re-coded to be able to sense the lower voltage of the LED trailer lights.
 
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