Factory towing wiring under seat?


Sprinter Westfalia #133
My 2004 Sprinter (Westfalia conversion) has the factory towing hitch. I'm trying to add an electric brake controller for a trailer. I've researched other posts about this, and the problem is that under the driver's seat, I cannot find the wires to attach the brake controller to.

My factory 7-pin harness works properly for brake/turn signals/lights. I am trying to find the "blue" wire that is supposedly under the seat, to which I will wire the brake controller output. I do not see the trailer light control module under the seat. For that matter, I also need to find the brake wire under the seat to connect to.

Power should be easy--I assume I can grab that from the fat red battery terminal under the seat, correct?

I'm attaching a pic of the underseat area. Thanks to anyone who can help me sort this out!



Erratic Member
The trailer light module is located in the pillar beneath your right rear taillight assembly.

There's a "how to install the trailer wires" document at: http://aie-services-2.net/Sprinter/
(along with other handy stuff)
Specifically: http://aie-services-2.net/Sprinter/T1N-trailer_wire_harness.pdf
(the module is item (C) in those instructions, shown just at the edge of Step 7.
In Step 8 it's shown attached to the harness with cable ties (B))

In Step 29, the wire you seek is shown as "LB-(A)" ("LB" means Light Blue)

p.s. it's all perfectly clear if you drink enough Schnapps ... just keep drinking until it is.
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Sprinter Westfalia #133
Thank you very much! I've seen this doc but somehow missed that crucial detail. It appears I'm basically looking in the wrong place and don't need to do anything under the seat.

I'm new to brake controllers, but if I understand this correctly, I should be able to find the other end of that light blue brake control wire tied up and disconnected in the driver footwell area. So I will need to manually connect my brake controller to the brake switch and this blue wire (plus of course battery power and ground), and I should be able to get a brake signal at the 7-pin hitch jack. I will try it tomorrow and hopefully find the wire where it is supposed to be.


Well-known member
I would think it is under the seat. The factory wiring setup shows that. It is a light blue wire. It could be up under the hood. The three wires that reach past the lights are +12v all the time, Ground, and the brake wire in blue.


New member
I installed the MB euro factory harness in 2010. The trailer turn signal module was installed under the driver's seat on the bottom right side of your pic. It's one of the large rectangular relays. Mine is in the aft outboard position, if that helps. It's not a brake control module, just a relay for the trailer lights.
I can post the install instructions if you need them.



Sprinter Westfalia #133
So I had the chance to go out and check for wiring. I could find nothing under the hood by the battery. I also did not see anything obvious by the driver's foot well. Hmmm. I'm going to go back out in a bit and see if I can trace where the rear jack wiring runs to--I am thinking that it is possible that this was wired off the tail lights and that's it--no wire towards the front. I know for sure it is factory installed from Germany because it is on the data card for the vehicle. So theoretically there should be something up front but I wonder where.


Sprinter Westfalia #133
Update: Nothing quite matched up with my Airstream Westfalia Sprinter. These have some differences with the more mainstream Sprinters and the trailer wiring appears to be one of them. I ended up running a new 12 gauge brake control wire under the vehicle along other wiring. It was pretty easy actually, although without a jacket around the single wire I am a little concerned about longevity. The wire is at least nicely tucked underneath.

In doing this I essentially eliminated the underseat area entirely from the controller. The controller I installed is the Hopkins Insight. It has three parts. The "brain" is mounted along the center console tucked up high above the accelerator pedal. The tiny display is mounted near the hazard light switch for easy viewing while driving. The manual override is on the cluster just behind the wiper stalk. The flexibility of this system seems great so far. We'll see how it does in practice.

Thanks for all the help.

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