I have a 85 380SE with 178k. It has a rough idle and gets rougher when you put it in gear. Drives fine on the highway.

Put new plugs in it and cleaned the wires. They had some corrosion on them. Talked to a MBZ tech in my area and he said could still be the wires or a vacuum leak near the intake.

How do you check for vacuum leak? Would like to check that before getting new wires. Any other ideas are welcome.