The A6 is redesigned for 2019. Overall, the back seat hasn’t really gotten any friendlier for car seats – there is still seat belt overlap, still rigid buckles for all 3 seats, still a bump at the seat crease for all 3 seats, and 2C is now narrower and still does not accommodate a narrow booster. This is not a vehicle for families looking for 2 side-by-side or 3-across in the back seat. 
2012 saw a slight change in the head restraint design for 2D & 2P – otherwise this back seat is (sadly) the same as it has been since 2005.
The buckles for all 3 seats are rigid and flush with the vehicle seat cushion – and worst of all, 2C is super narrow and the buckle for 2C is directly in front of 2D’s buckle. This makes 2C useless for most seats. We’ve found a few rear-facing seats will work in 2C – like the Chicco Fit2, Cybex Aton2 (without the load leg) and the rear-facing Clek Fllo. We haven’t had any luck with forward-facing seats or boosters in 2C.
There was a slight change in the head rest design for 2009 – where they now protrude more over the vehicle’s seat back. Otherwise the back seat is identical to the 2005-2008 in every other way. 
The A6 was redesigned for 2005. Sadly, it wasn’t a good change. The buckles for all 3 seats are now rigid, and flush with the vehicle seat cushion – and worst of all, 2C got narrower and the buckle for 2C is directly in front of 2D’s buckle. This makes 2C useless for most seats. We’ve found a few rear-facing seats will work in 2C – like the Chicco Fit2, Cybex Aton2 (without the load leg) and the rear-facing Clek Fllo. We haven’t had any luck with forward-facing seats or boosters in 2C.
Starting in 2003, lower anchors come factory installed in the A6 sedan for 2D & 2P. Otherwise the back seat is identical in appearance to the 1998-2002 back seat.
The buckle for 2C is flexible, and 2D & 2P are mostly rigid. All the head restraints are adjustable (they can tip forward and back as well) and may be removable. The bump at the bight in all seating positions (most prominent in 2D & 2P) will interfere with the secure installation of some seats – especially infant seat bases. This vehicle has a wide, flat back seat that is ideal for fitting multiple car seats.
Starting in 2000, tether anchors come factory installed in the A6 sedan. The tether anchors are located under rectangular plastic caps on the shelf below the rear windshield. Otherwise the back seat is identical to the 1998-1999.
The buckle for 2C is flexible, and 2D & 2P are mostly rigid. All the head restraints are adjustable (they can tip forward and back as well) and may be removable. The bump at the bight in all seating positions (most prominent in 2D & 2P) will interfere with the secure installation of some seats – especially infant seat bases. This vehicle has a wide, flat back seat that is ideal for fitting multiple car seats. 
1998 saw the addition of two important safety features for 2C: a head restraint and a shoulder belt. The buckle for 2C is flexible, and 2D & 2P are mostly rigid. All the head restraints are adjustable (they can tip forward and back as well) and may be removable. The bump at the bight in all seating positions (most prominent in 2D & 2P) will interfere with the secure installation of some seats – especially infant seat bases. This vehicle has a wide, flat back seat that is ideal for fitting multiple car seats.
Starting in 1992, tether anchors can be retrofitted in this vehicle for all 3 seats. 2C only has a lap belt, and no head restraint – while 2D & 2P have head restraints & shoulder belts.
The lap belt in 2C may be the type that is unusable for most car seats without the addition of a belt shortening clip. To see what type it is, do the following: Pull the lap belt out (slowly) all the way, and then let it in a few inches – and now see if you are able to pull it back out all the way. Does the belt let you make it longer – or will it only let you make it shorter?
If it will let you make it longer, then this lap belt can not be used to install a car seat without a belt shortening clip. A belt shortening clip is NOT the same thing as a locking clip. Please find a certified child passenger safety instructor in your area if you must install a car seat in this center seat.
If it will only let you make it shorter, this belt can be used to install a car seat – and after routing the belt through the proper path on the car seat, you will pull the lap belt out all the way to put it into its locking mode, and then while applying pressure to the car seat you will feed the lap belt into the hole where it emerges from.
The Audi A6 was first introduced in 1990.