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2003 Cayenne S

So my wife's pepper had a fishbowl in the passenger rear taillight for awhile now. I guess long enough that it shorted something out as the running lights no longer have power.

I have 12V going to every pin in the connector except the common orange wire for the dual bulbs in the bottom of the tail light. Brake lights, turn signal, back up light work just fine. The fuse on the passenger side panel is in proper working order.

I believe there is a controller box for the taillight somewhere in the rear cargo area (not the tow controller, the actual taillight controller). I am guessing that the driver for that power line has shorted out since I cannot get 12V on that one single pin (I think it is pin 3; grey/red strip wire).

I have the rear cargo area torn apart looking for the control box with no luck finding it. The cable harness branches and comes back together in so many places I cannot identify where that Grey/red stripe wire gets power from.

Does anyone have knowledge, or a graphic of where that control box is? The wife is getting pissed that she can't drive the truck at night for fear of getting pulled over by the police.

Any help is greatly appreciated!!!


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  • 6 years later...

I hope that someone benefits from my experience with a not working tail light on a 2005 cayenne. As I use this forum often to give me ideas on how to deal with issues it is only fair to contribute.  So one day I noticed that my left hand side tail light was not working. All other lights on the car were fine including the left fog light which shares a tail light bulb and other lights in the said cluster. 


My my immediate reaction was two bulbs blown at once not impossible but unlikely. Checked fuse, bulbs and fine. So after a couple of days of head scratching continuity testing and so on I took the decision to run the car down to the local Indy dealer and book it in.  


They said. If the loom has gotten wet before, and it was soaked in the left hand passenger foot well when I bought the car, then I probably have corroded connections which need fixing. And potentially its on both sides of the car.  Quoted 3 hours work to tear into the left side and repair. One hour if I remove seat and pull up carpet for them to go at it which was my preferred option.  The wet problem had been solved a year before and was due to overflow from blocked drains in the firewall as is all too common. 


So I tore into the left seat and carpet and exposed the loom. At this point I could not resist going further so I removed the cloth sleeve carefully and exposed I think 6 hot crimped and taped up wire connections. When I unwrapped the tape I found green corroded connections. The fatter wires were green but connected. The smaller wires were not so great with one of the connections falling apart in my hands. I cut them all one at a time back to good wire and soldered and double heat shrinked all. Good job I thought. 


Light still not working. So I followed the loom past the battery and into the rear left passenger floor.  Looking for more suspect connections.  I didn’t find any. So I then moved to the right side on the basis that the Indy told me both sides could be an issue. Seat out and carpet up. Opened the loom. There was one connection near the gas pedal which was fine. In fact it was pretty evident that this side had not been wet before like the passenger foot well. I found another connection that looked like it had been professionally repaired before with glue filled clear heat shrink tube.  That was all good. So nothing obvious.  So I continued rearwards. Cutting the covering tape to expose the wires. I found nothing and it was like new under there.  


At at this point I am getting very disappointed. I’ve now read every story online I can find about touaregs, Audi’s vws and any thing else remotely relevant. With many parts ripped out I was reluctant to do more and resigned to go to the dealer  However I knew that there is a control module under the dash. That looked ok. I bought a replacement light switch out of desperation and I bought a rear convenience control unit also out of desperation - This unit sits next to the CD player over the right rear wheel. 


So I then removed the back seat so I could get to the rear control unit. It’s a total pain in the *** to get to and while trying not to break things...  while doing so I found that one or two screws here and there were missing. So now I am potentially following a prior path.  


I found a westfalia unit which I guess is for the tow hitch, another small unit which I can’t recall what it is and the rear control and convenience module. Now I can clearly see that the suspect unit is not screwed down properly and the mounting bracket is also not fixed properly. Hmmmm. So at the risk of screwing up the electrics completely I took the replacement unit I had bought and changed it out for the original. Alarm went off. Rear door mechanism went mad for a minute. Stopped alarm with remote and door mechanism stopped on its own.   Tried the lights....... working!!  


Ok when I lock the car the fuel flap opens and the rear hatch doesn’t power close and the interior lights don’t go off on close but this is no doubt a coding issue with the replacement unit. But at least I have now established it’s not a break in one of 200 wires running through the vehicle. Milestone reached.  When I bought a replacement module I couldn’t read the exact number of the unit and it’s different. It’s from an earlier cayenne. Anyways I have since ordered one with exactly the same numbers so maybe this will work without coding.  Don’t know yet as I write this. Otherwise I will put the old one in get the code read and copied to the new one. 


Great. What a journey. Then I opened up the busted module to have a look at this mini pc that controls the rear of the car and other stuff. There is a small dried up pool of green dust in one corner. It’s gotten wet. So it’s now broken. Ok. Why has someone been in here before and wet from where?


Now when I bought the car 18 months ago the rear window washer just blew water out from under the third brake light. When I investigated this I found the spray nozzle blocked and the hose had therefore disconnected and was free under the rear spoiler just blowing water down the glass. I took the nozzle apart and got it working. But then it got stuck again and did the same thing. Then I found that you can buy a new one from the dealer so I did. Fixed. 


Also so when I bought the car the headlight washers were broken and the jets missing etc. so I fixed them with new parts. But. Even now when I fill the washer reservoir to full it slowly over 24 hours weeps out of the washer jets. So after losing lot of washer fluid I started using plain water. 


I have ave not fully completed the correction or investigation as I write but either the rear washer supply hose leaked some how when it was blocked and wet the module or in freezing weather the same occurred I don’t know.  But this is the only potential source of water in this module save for pouring a drink over it. 


And why was someone in this area of the car before. I don’t know but I expect I will soon find a weeping pipe or connection near this unit. 


That is the end of the story as I now have my lights and start to put it all back together. I get the new module which might work completely tomorrow and I’m going to hunt for this water line to the rear which I think is the culprit. 


So for anyone who might be experiencing some strange electrical issues in the rear of their cayenne or similar I hope this helps. 

The wet loom is clearly a potential cause but not in my case. And i gather that on the left side loom connections if they are bad you will get error codes relating to random functions of the car which I never had. Having said that I did have the speedo go up to 100 mph twice in a year when stopped so this might have been fixed now? Every other function or switch on the car is good and has been. Just somehow lost a tail light. The clue that pointed me to the back of the car was that the license plate lights are on the same fuse and they were working so I determined in the end that power was getting through and something was not doing it’s job are the rear.


And why was someone in their before?  I guess for the same problem before my time.....these systems are potentially so reliable but pour water on them and ....ever spilled a drink on your laptop..   computer says no.


i don’t mind fixing cars but this really did my head in. Maybe this story will save someone the pain of ripping the car to pieces as I have done. Can’t wait to get going again and spending massive amounts of money on petrol. Not only have I saved the dealer cost of repair I have not run a gas guzzler for weeks.  But all said and done it’s a great car to drive. Waiting for the next surprise.........




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And just to update. I have put another convenience control module in the back with exactly the same numbers as the original and all systems working fine. No coding required. Problem resolved. Locking closing lights ets all good. ( so far). Off to the gas station....

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  • 1 year later...

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