Chuck has a great list. I replaced my Idle control box '86 420SEL (passenger footwell) and still had occasional high idle problems; I took a clean cloth and WD-40 and cleaned the throttle body where the round air plate fits, and problems stopped. I also replaced the injector seals later, anyway. Then I found a vacuum leak in the climate control. Air leaks = idle problems. I dread replacing the rubber seals under the throttle body, but I know I'm going to have to sooner or later. There is a ground on the firewall, you can see it if you pull the instrument cluster. I did not check to see if it was accessible from below.
If you like puzzles and mystery novels and have an alternative vehicle, you'll make a great Mercedes-Benz hobbyist.