This is one of those DIY FAQs that has lots of great information and tells you how to fix broken stuff, but it fails to explain the real reason the switch fails and doesn't provide any real technical info. If you want to know more let me illustrate the workings of the switch. The following will allow you to get a failing brake light/pedal switch to work at least temporarily.
Audi/VW has had numerous problems with electric switches since the early 1980s. Having owned several Audi 100s/5000s, I have become an expert at taking the electric window control switches out of these cars and cleaning the poorly designed contacting surfaces that are at fault. Sadly, the brake light switch (which FYI is the same as the brake pedal switch, just 2 different pins on the same switch) on my 05 Jetta is still based on the same design and therefore suffers the same problems.
All newer models have a purple bodied rev. E switch. This switch is actually DIFFICULT to break and in all likelihood can be cleaned / repaired easily. The switch can be readily removed in its unlocked position. You may choose to reset the plunger or you may choose not to, I haven't found that it makes any difference.
1) The plunger part of the switch (which comprises most of the switch) doesn't include any components likely to break. The actual switch part is controlled by a small nub from the plunger. Therefore getting at the innards of the switch is EXTREMELY easy. All you have to do is GENTLY pry the two tabs apart keeping the pinout part of the switch in position (circled in red in the picture below). Then gently push the pinout portion backwards and out.
2) The switch is designed to be CLOSED on pins 2-3 (inner two pins, called the Brake Pedal Switch F47) and OPEN on 1-4 (outer two pins, called the Brake Light Switch F). This is to allow the ecu to be able to check if the switch is functioning, ie have one closed circuit at all times. This is great for the ecu, but not so great for the 1-4 circuit as it tends to arc as the circuit is closed. This leads to corrosion/pitting and eventual failure of the 1-4 part of the switch. Specifically, in the picture below, the contact area is circled in red.
3) To temporarily fix this switch, all you have to do is clean the contact area. The best way I have found to do this is to get a very small piece of sandpaper, lift the switch apart, place the piece of sandpaper in the contact, GENTLY press the switch closed, and sandpaper the contact back and forth a few times. This will make the switch work for a significant amount of time, anywhere from a day to a year depending on your skill and luck, as well as the local environmental conditions (humidity especially will kill these types of switches). In any case it will allow you to get your brakes working immediately on a manual model and it will allow you to shift an automatic model without downtime.
You can check the operation of the switch by using an ohm meter or alternatively a Vag-Com. The Vag-Com is particularly useful for repeated and functional testing with the switch installed.
Modified by DrSmile at 5:44 PM 9-4-2006
Modified by DrSmile at 8:47 PM 9-4-2006