Repaired the electronic idle speed control unit on my w126 500se two weeks ago. For a helpful picture of the unit look at the parts catalogue in Fastlane.
The location of the unit varies from model to model but in the RHD 500se the unit is situated above the steering shaft and behind the instrument cluster. It is held in place by a self tapping screw which is well nigh impossible to remove without a flexible screwdriver. Once removed it is best to replace with a suitable cable tie.
Faults on the electronic unit are generally dry joints which can be easily picked out using a magnifying glass or other magnifying viewer. Simple repair with a soldering iron.