I thought I'd update this thread regarding my parking light staying on issue.
The problem began one evening on my way home when my left turn signal suddenly stopped working. When I got home, I pulled out the headlight to change the turn signal bulb. I noticed the internal headlight wiring, especially that to the parking light, was bare or cracked.
I pulled the turn signal bulb and tested it but it was not blown. Very strange!
I then referenced the fuse box and discovered a 10A fuse for the left turn signal was blown. Also very strange.
I replaced the fuse and regained my left turn signal. It was then I noticed the parking light on the left side was still on. WTH???
I pulled the parking lamp and called it a night. I did more testing the following evenings and weekends as I had time.
Around the same time, I started getting a "Fault Running gear workshop" error. VAGCOM said I had a faulty sensor in my right rear air shock. Clearing the codes did nothing, they immediately returned. So now I had two problems. Ug!
For the shock, I popped off the connector from the shock and checked continuity between each end of the wires and the ride height control module below the CD changer in the back. All were fine. This lead me to believe that at 200K miles the sensor had failed. It's apparently not a sensor that can be replaced. The whole $1500 shock is required, and you really should replace both rears at the same time. Ouch!
Then I moved onto the parking light issue.
I went ahead and completely replaced all the internal headlight wiring in the left headlight. Amazingly the right headlight wiring appears completely fine and still pliable.
All testing I did was at the large plug the headlight unit plugs into. I compared left to right and with the headlight removed, key out, light switch turned off, there was constant power to the one pin that feeds the parking light on the left light but not the right.
I referred to the wiring diagram (thank you Bentley Online) and saw that other than one other connection, the headlight plug gets it's power through the J519 comfort and convenience module. This clarified to me the problem was in the J519.
I found a used module on Ebay for $60 and when it arrived, I used my VAGCOM to record all my current settings so I could reprogram the new J519.
I popped the old out and plugged the new one in, reprogrammed it and tested. Light was still on! ARRRRRGGG!!!!
That was it. Between the airshock sensor and this light I was at the end of my troubleshooting ability, so off to the dealership I went.
The rear shocks were replaced, and the the technician initially said I needed to change the headlight. I said no, please pop out the headlight and test the plug. He did and agreed the J519 was the problem instead.
I provided the Ebay J519 (Dealer offered to order one for $900) and the tech installed it. Somehow he was able to get it programmed and the parking light went out.
Lastly, while there, my CEL came on when the technician pulled it into the repair bay. The VW dealership factory scan tool said it could not communicate with either ECM so they could not check the CEL. The technician had a generic OBD tool and was able to read the slave ECM to see there was a glow plug error on cylinder 1, but he could not clear it.
At this point the original repairs were done so I offered to bring my VAGCOM over and clear the code. I'd seen the code other times in the past when it was especially cold out.
I was surprised the tech using the factory "VAGCOM" could not communicate with the ECM but it turns out that because the vehicle is so old (2004) the software used by the tool has been updated to the point that these older cars are less and less supported by it. Once I connected my VAGCOM, I was able to communicate with both ECM and clear the code.
So, lessons learned are:
Crumbling wiring in your headlight that blows a fuse leaving the parking on permanently, probably means you need a new J519
Talk your dealership into buying a VAGCOM so they can keep working on your old V10
I now have all the pins and wiring to rebuild the insides of a Touareg headlight so if you need a new wiring harness in yours, let me know and maybe I can help.