My Euro 107 had the Idle Speed Control module behind the glove box on the left. It is marked as such. One bolt and the connector and it’s in your hand. I couldn’t get the cover off so I replaced it. I know I bought aftermarket parts and got an idle speed control and Air slide valve for about $400 (total). The air slide valve (on the intake manifold) makes a buzzing sound. A neighbor has an ’85 500SEL and he has the new MB part. His make a hum. The MB part seems to run smother but it was the difference between $400 and $150 for the part.
’85 500SL Euro