I have a B4 2001 and from reading other posts, the procedure should be about the same. Pry off the small circular cover from inside the boot in the general vicinity of the top of the light assembly. Mine was about the size of the fifty cent piece. There is a bolt/screw underneath this cover. Remove it, taking GREAT care not to drop it behind the boot fascia. Once it's removed, you should be able to wiggle the entire light assembly from the outside. My model assembly was held on by the single bolt/screw and two 'plastic plugs' that fitted onto the front-most part of the assembly. These two 'plugs' simply pull out of the car frame (not screwed in) and come straight out with some gentle wiggling of the assembly. The light bulb 'frame' for the bulbs clips into the assembly and is easily removed.
Replace the brake pads.
Pull the carpeting behind the brake light. Unclip the assembly, replace bulb.
As of 2014, the cost for a Audi A4 brake light is between 12 and 15 dollars. This light can be purchased at stores like Auto Zone.
The brake light in the Audi A4 display means there is an issue with the brake system. The most common cause of this warning light is low brake fluid.
This site has step by step instructions (with photos), perhaps the A4 is similar. http://audidiy.blogspot.com/2010/04/how-to-replace-brake-light-bulb-on-audi.html
you go.... and replace it!
By going to the nearest service!
You need to replace the break pads or you can bodge it up by connecting the two wires
Warning light symbols with explanations for 2003 Audi a4
My brake job on my A4- rotors and pads- $473.
Faulty ABS unit .
replace brake line