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Discussion Starter · #1 ·
This DIY is for 7P Tregs that do not have the factory tow preparation (basically all MY15 onward Oz delivered cars).
Ok so I got my tow bar installed in my shiny new treg and went to plug by tekonsha prodigy into the 4 wire plug under the dash, just like my old car, only to discover that the connector is no longer there.
The reason is simple, WV Oz have moved to the European 13 pin connector, and there’s no provision for service brakes in the specification for that plug, there are several “unused” pins that can be used for brakes, however there’s no standardised pin?!?!?!?.
I had the genuine VW bar and controller installed, although this DIY should work with any setup, as it’s bypasses the tow computer altogether as the tow computer has nothing to do with the brakes anyhow.
The dealer fitted a Narva branded 13 pin to 7 pin flat adapter that bolted to the towbar, it was actually a very nice neat install and looks and works fine.
So here’s what you’ll need.
1) Brake controller (obviously) I’m using my trusty Tekonsha Prodigy P2.
2) RED and BLUE wire, I just bought the standard 10amp stuff from Super Cheap Autos, 7m roll was about $6 each roll.
3) Soldering Iron, heat shrink and heat gun, various crimp connectors.
4) 20AMP self-resetting circuit breaker, I elected for one in the standard blade fuse package, as they’re smaller, neater (and cheaper) than the normal twin pole ones. Got it from Jaycar for about $5.
5) Set of Torx drivers, misc hand tools, a multi meter, some cable ties, silicone sealant and some panel/trim pullers.
All up it took me about 4 hours, if you’ve never done anything like this before, I’d suggest allowing the whole day. You’ll also need to be reasonably ok with wiring stuff up and have some experience soldering as well. The trim panel are tough to remove and the clips break easily, so take care.
So here we go.
1) Remove lots of trim (this part is well described elsewhere so I won’t go into great detail about it). Remove the 4 silver luggage lock down rings, and the side supports they hold down. Remove the lock carrier cover (big silver bit at back of car) by gently prying it up. Remove the upper rear “D” pillar trim by removing the torx bolt and then carefully prying it toward the back of the vehicle. Remove the “C” pillar trim, you can just carefully pry this away, no need to remove the seat belt, just let it hang there around the belt. It sort of swings out from the top. Pry out the lower wheel housing trim. You don’t need to fully remove this, just enough to get at the spreader clip for the side panel. Next remove the right rear lower luggage compartment side trim panel. This one has 2 small “spreader clips” that are unscrewed with a T15 or T20 driver depending on if they’ve been out before. My car’s were rounded a bit by the monkey who installed the tow bar, but they eventually came out. Unclip the light wire, and power socket wire, and 2 wires for the seat release buttons and the load height adjustment (if you have air suspension like me). The panel should then lift out. Your car should now look like this. I’m pointing to the tow computer location. The small silver box in the upper right is the rear view camera unit and the larger silver box in the lower right is the MMI (stereo) unit.
Vehicle Auto part Vehicle door Automotive exterior Car

In this picture you can see the tow wire harness passing through the big rubber grommet (that the monkey didn’t bother pushing in properly). There are also a couple of very handy unused rubber grommets that are perfect for our purposes in this section too.
Wire Electrical wiring Tire Auto part Technology

2) Next you’ll want to remove the 4 screw at the bottom of the bumper bar. It’ll allow you to move the bumper around a bit and give you some clearance. I’m pointing to one that’s partially out.
Vehicle door Automotive exterior Bumper Vehicle Auto part

3) Next unbolt the 7 pin socket from the tow bar. Mine was secured with screws with nylock nuts. When I put it back I reversed them so the screw head was on the bottom, it was heaps easier to deal with that way.
Vehicle door Automotive exterior Bumper Auto part Trunk

4) Pull the 7 pin socket out and unscrew the cable retainer. The socket then just opens up and the bottom cover comes off. Here you can see all the wiring, the wires we’re going to use and the Red wire (brake lights) and the blue wire (trailer brakes). Curiously in this adapter the blue brake wire was connected to Pin 13 on the Euro plug? Not sure why, but it doesn’t matter we won’t be using it.
Electronics Technology Electronic component Electronic device Electrical connector

5) Pop out one of the rubber grommets I showed you before, and poke a small hole in it. Unwind ALL your red and blue wire, and pass both wires through the hole in the grommet. I passed about 1.5 metre though to start with.
Wire Electrical wiring Light Cable Electronics

6) Next I used an electricians wiring snake to pass over the exhaust and over to the tow hitch. Tape your red and blue wires to the snake and just pull them through. It’s a bit fiddly, but not too hard. Don’t pull the wires tight yet, we need some slack in them.
Automotive exterior Bumper Automotive tire Tire Vehicle

7) Ok now we need to modify the existing 7 pin flat plug wiring. The blue brake wire is easy, just loosen the little retaining nut, remove the existing wire and replace with your blue wire. The red wire is a little harder, first unscrew the retaining nut and remove the existing red wire. You’ll then need to very carefully strip back a small amount of the insulation on the exiting red wire, wrap your red wire around it and solder it up for a good connection.
Electronics Wire Electronic engineering Electrical wiring Technology

8) Now put heat shrink around the join on the red wire, and also over the old blue wire as we don’t need it anymore. You’ll then need to file or drill a hole for our new wires to exit the 7 pin connector. You can see in this picture how I did it. (note the old unused blue wire with the heatshrink over it’s end. The diodes also have something to do with tying the left and right brake lights on the euro plug to just one light on the 7 pin plug, but it doesn’t seem to work on my car, I always get the check left brake light error.
Electronics Electrical wiring Wire Technology Electrical connector

9) Put the 7 pin socket back together and bolt it back to the tow bar. Put a small dob of silicone sealant around the hole you drilled so no water can get in. Next pull your wires a bit tighter. You’ll need to zip tie them to the existing harness where you can. Take special care where the wires run over the exhaust, you’ll need to feel around up there to find the existing wiring harness so you can zip tie your wires to them. Take up the slack from inside the car, and push the rubber grommet into place. I put a small dob of silicone sealant where the wires come through. Here you can see my wires and grommet in place correctly. The factory wiring harness (that I put in properly) is up in the top left (a bit hard to see)
Wire Electrical wiring Fuel line Auto part Cable

and the same wires coming up from below the car zip tied to the original wiring harness
Wire Electrical wiring Audio equipment Cable Technology

and here’s the wires coming out of the back of the 7 pin connector.
Automotive exterior Vehicle door Auto part Bumper Vehicle

10) Now you can feed your red and blue wires up to the front of the car, just run them under the plastic kick panel that runs along the floor, it’s not too hard, a flat bladed screwdriver helps. Here is a picture of the two wires coming out near the bonnet release.
Trunk Vehicle door Auto part Vehicle Car

11) Now it’s time to mount the controller itself. Remove the rear lower trim panel under the dash, unclip the light, and push the diagnostic port through by pressing the 3 retaining lugs at the same time (you can see the diagnostic connector hanging loose in the picture). I used the pocket mount (as I normally remove the controller from the vehicle when I’m not using it). I mounted it under the dash on the far right hand side, fairly much as close to the door as possible. Make sure you don’t obstruct the knee air bag!
Vehicle door Vehicle Car Automotive exterior Trunk

12) Now we need to make our wiring harness. The prodigy harness has 4 wires, 12v (black) earth (white), brake signal (red) and brake output (blue). You can see in this picture I’ve put a ring crimp connector on the earth and put my circuit breaker in the 12V line. I’ve soldered all the crimp connectors too (cause I don’t trust them) and used heat shrink around the circuit breaker connectors.
Wire Cable Electrical wiring Electronic device Technology

13) Here I’ve soldered the red and blue wires we bought from the back of the car to the prodigy’s harness and heat shrinked them up. I’ve also soldered a flat blade crimp connector to the constant 12V as we’re going to grab 12V from an unused fuse location in the right hand fuse box.
Wire Cable Electrical wiring Networking cables Electronics accessory

14) Next we need to connect the earth and constant 12V. Fortunately there’s a great earth in the right hand fuse box on the screw that holds the dash to the dash support (upper left of photo). Also push your blue blade crimp connector into an unused fuse location that has constant 12V. There were 3 in a row that were unused with constant 12V on the bottom row of my fuse box, but just use your multimeter to find one. (make sure the car is turned off as you need constant 12V for the prodigy, not switched).
Electronics Technology Electronic device Machine Electronic component

15) Next refit the lower dash panel (don’t forget the light and diagnostic connector) and then pull the prodigy harness down and through the back of the mounting pocket. Make sure you route the wires so they don’t get caught on anything. Here’s where I ran the wires around the lower dash panel
Games Auto part Technology Vehicle Automotive exterior

16) Replace the fuse box cover, and plug your controller in. The prodigy should light up and flash “N.C.” ( not connected) on it’s display. That’s good it means you got the 12V wired up correctly. Next go and plug in your trailer and make sure the prodigy shows “c” (connected) in it’s display. Have a helper press the brakes and make sure you can hear the “buzz” of the trailer brakes and also check the prodigy display shows some numbers. Test the brakes as per the controller’s manual.
Vehicle Auto part Car Automobile pedal Family car

17) Refit all the trims you took off. Don’t forget to reconnect the wires in the luggage panel, and remember the lock carrier cover goes back before you put back the rest of the floor (doh!).
18) Grab yourself a cold beverage and congratulate yourself on a job well done!

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Well detailed post Darrin. Bit curious why you didn't connect into the 13pin Euro plug as that is probably inside the vehicle and is connected to all the wires you used in the 7 pin?

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80 Posts
Discussion Starter · #4 ·
Hi Schootsie
I considered tapping into the euro plug connector inside the vehicle, but decided against as I didn't want to mess with that harness, as in the event of a fault, it'd be a drama to diagnose and VW would simply say "not our problem, you hacked the car's wiring up".
This way you can just unplug the 13-7 pin adapter for troubleshooting or even replace it with a non-modified one to take the controller out of the system, but it's certainly an option I guess, not hard to figure it out with a multimeter.
Towed my ancient Coromal on the weekend, and all seemed good.

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1 Posts
Thanks for the detailed and very well presented information. It was a life saver for me and saved a lot of messing around to find a live connection on the fuse box instead of trying to route a cable to under bonnet pos terminal (near impossible). Cheers
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