The part that you're looking for is a vacuum servo with a builtin switch. It's covered by the white thing in this pic:
http://bos-engel.com/benz/servo.jpg
You may want to consider a new part though. Your car is now 16 years old and the servos have been operated thousands of times. The rubber diaphragms are getting hard. I doubt that a new part is that expensive for the amount of effort to get at it.
