I worked on high idle issue on my 1991 560 SEL for 5 months and finally fix it for nothing.

To answer your specific question, the idle control unit on your passenger side. Take teh carpet off and you should see a black cover and the unit is behind that. However, if you get a good voltage on your idle control valve, your ICU shoudl be o.k.

The very first thing is to clean the idle control valve. Do a search on that one and you will find several well written steps.

I went over everything, even took the car to MB dealer, and could not solve my high idle issue. Finally, I just took the advice and stuck a core with a 5 mm hole into the ICV and that was 2 months ago. The car ran beautifully ever since.