Based on a 1999 A3 1.6 Sport 5dr

On the driver's side (UK off-side):

1. Locate the light cluster cover on the rear of the light.
2. Push down the 2 retaining lugs on the cluster rear cover.
3. Prise the cover backwards to break the seal and lift out diagonally backwards.
4. Locate the sidelight bulb holder (this is under the main/dipped headlight bulb).
5. Holding on to the black bulb holder, not the wires, pull out the bulb (This is quite fiddly as there isn't much room to get your hand in!).
6. Remove the bulb from its holder and replace with a new one being careful not to touch the glass directly - a cloth should be used.
7. Reinserted the bulb holder by simply pushing it back in until it clicks.
8. Replace the cluster rear cover by locating the bottom first and clicking the 2 top retaining lugs back into place.

On the Passenger side (UK near-side):

Process is the same as for Driver's side only the battery is in the way. I believe the options are probably to remove the battery from its housing, or remove the light cluster unit altogether. Anyone else have any ideas?

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....

• Open the tailgate!
• Look at the tailgate opening and you will see a plastic strip that protects the frame (it goes clockwise from 3 O'Clock to 9 O'Clock)
• Located in the above mentioned plastic strip you will see two small round plastic cover disks (one on each side of the tailgate opening).
• Use the flat blade screw driver to pry open the left or right hand disk (depending on which light cluster you want to remove)
• You now have a hole - if you look inside you will see a screw with a cross head and 8mm hex flats.
• I recomend that you unscrew this screw with the 8mm socket, but you can probably get away with a cross head screwdriver in an emergency (Note: you often find a combined cross head/flat blade screwdriver in the car's tool kit - take a look in the boot/trunk where the spare wheel is located - if you have one!).
• The above screw is quite long so keep on turning until the tail lamp unit moves freely where it meets the tailgate opening - but don't try to pull the lamp unit off yet. The screw DOES NOT NEED TO EXTRACTED so don't try to pull it out, it is captive in its hole.
• The next step, which is probably the most difficult is to "pop" the pins out of their sockets. These pins are located on the side of the lamp unit, but not visible from the outside of the car. To get an idea of where they are located imagine placing two fingers at the top front corner of the lamp, just below you finger is the top pin. Place three fingers at the bottom front corner of the lamp and the bottom pin is just above these fingers.
• Try waggling the lamp unit gently sideways off the car (but don't try to rotate it towards the front of the car or you will brake the pins and possible the side of the lamp unit as well). If this doesn't free the unit, try gently tapping the lamp on the side (black plastic facing into the tailgate opening). Still no luck? You are going to have to pry the lamp away from the chassis by placing your screwdriver between the body and the lamp adjacent to the pins mentioned earlier - but this is high risk, you can easily damage the painwork doing this.
• Once the pins have popped out of their holes you need to free the lamp from the harness. There is a small clip on the connector (which is located on the long black bulb-holder strip) which must be pressed to release the connector.
• Now you can work easily on the lamp unit. Look around it, there are 5 black lugs that fit into 5 red slots in the lamp body. Prise the red slots appart to release the lamp holder.
• Turn the black lamp holder over and you now have access to lamps.
• It's pretty likely that the one that's gone is the combined brake/tail light bulb, if this is the one you need to change, it's at the bottom of the bulb holder.
• Push, turn and pull - the defective bulb is history!
• Re-assembly is the reverse process above: don't forget to make sure the sealing rubber is correctly seated in the bulb holder before you click it back together again.

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