There is only 1 wire going to the sensor. It can only be floating or ground to chassis. When it is floating, light is off. When it is grounded, light comes on. It cannot get simpler than that.

The grounded signal may go through a computer to debounce it to stop the light from flickering often. The concept is the same for most sensor/switch detection.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.