Idle Speed Control Installation

I have a 1985 380 SE Euro Spec Benz. I took my car into the Mercedes Dealership because my car's idle was irregular. I checked the air slide, and it was functioning properly, and the mechanic at the dealership told me that my idle speed control was the culprit. He told me that it would be about $500 for part and installation. When I looked at the part that he was referring to it seemed that I would be able to install this my self. I checked with the dealer and was told that the part cost $291.50. How difficult is the install? How do I do it?

Thank You in Advance for Any Advice!