Have a new problem with my 2008 A6 3.0 FWD. In the morning or when the car is cold, can't get the gear shift out of park without running car for 10-15 minutes. Dealer is clueless and originally thought it was some minor corrosion. Took car home only to have same problem next morning. Then they wanted to replace gear shift mechanism for $1400. Don't think they know what they are doing and could use some help in sorting this out. In PA but may need to go to my original dealer in NJ, but that is 3 hours away.

 

Most obvious issue with not being able to get out of Park is the Brake light switch. So far I've only seen those things go bad and not work when the car warms up though.

Easy test - when it is cold and refuses to shift out of P, press the brake down and see if the car indicates you did hit the brakes. Without it knowing you are using your brakes it will not allow you to shift.
On my older A6 there is an indicator light next to the shifter when I use my brakes - not sure if you have that too but I'm certain there is a light somewhere when you use your brakes.