If you have a 2001 A4 Avant, you can access the bulbs from inside the rear Cd changer and road jack compartments, using only a screw driver.
You will need to remove the three screws holding the CD changer harness to access the plastic lamp unit on driver's side. Once it's out you can pinch to release the lamp unit and simply swap out the bulb. For the brake light, you will need a Sylvania/Osram 7528 which has a dual filament. Note that the little raised metal bumps on the side of the bulb base mean it only goes in one way before giving a little twist to lock it in place.
On the passenger side, simply remove the jack from the compartment and you can reach the lamp unit. Pinch the white plastic tabs to pull it in and you can swap out the bulb without needing any tools.
Here's how to do it if you have an A4 Avant up to 2004:-
To change a bulb on the the A4 Avant model year 2004 you have to remove the entire tail lamp cluster, disconnect it from the wiring loom, pull off the black bulb holder plate, then you can finally get access to the individual bulbs to replace them. The process is pretty easy, but the owners manual recommends that you take the car to a workshop. If you can handle a screwdriver and open a refrigerator door you can probably manage this job yourself. If you don't like getting your hands dirty or are affraid of breaking something then let the nice people at Audi take your money!
The description below is quite long, but hopefully clear for even the most inept weekend mechanic.
You will need some very basic tools:-
1) A small flat blade screwdriver
and
2) An 8mm socket & driver (or a cross-head screwdriver - I will explain below)
Process:-
Each tail lamp unit is held in position by a single screw and two short pins (they are brittle - so be warned). We need to undo the screw and pop the pins (which are located on the lamp unit) out of the holes in the chassis. So....
So there you are - "money for old rope"! Good Luck.
Also see the photos on this link to see the exact location of the pins and how the light cluster comes away from the car.
http://www.jmaguire.com/blog/2006/02/05/stoplight.HTML
The 2011 Audi A3 has 34.8 in. of rear leg room.
The 2011 Audi A3 has 52.8 in. of rear shoulder room.
The 2011 Audi A3 has 36.9 in. of rear head room.
The 2013 Audi A3 has 36.9 in. of rear head room.
The 2010 Audi A3 has 52.8 in. of rear shoulder room.
The 2013 Audi A3 has 52.8 in. of rear shoulder room.
The 2012 Audi A3 has 34.8 in. of rear leg room.
The 2012 Audi A3 has 52.8 in. of rear shoulder room.
The 2013 Audi A3 has 34.8 in. of rear leg room.
The 2012 Audi A3 has 36.9 in. of rear head room.
The 2010 Audi A3 has 36.9 in. of rear head room.
The rear track of the 2007 Audi A3 is 4 ft. 10.8 in. (58.8 in.).