One problem related to wiring has been reported for the 2005 Audi A6. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2005 Audi A6 based on all problems reported for the 2005 A6.
The left drivers side break light/signal will stop working intermittently. I have noticed that it tends weather dependent. I have read Audi forums whom have said that the electrical wiring when exposed to heat (70 degrees fahrenheit and above) will short causing the led break/signal light to not work. My mechanic stated that it is also because of a faulty "light control module + wiring. " due to the intermittency and high cost of the job it has not been something that I have been able to fix. I am afraid that eventually none of the tail lights will work or even after repairing it, the same issue will arise. I first noticed this in may of 2015, however it was fine through the fall and winter. I again noticed it randomly through the summer months (June-Aug).