I just fixed my rough idle last week. In my case it was a vaccum.

I know most shops will automatically change the wiring harness when these cars come in with idle problems.

After changing the wiring harness yourself, I'd start with the simple stuff....fuses, vaccum hoses, connections, etc. Then do some of the inexpensive DIY stuff....change the motor mounts, clean the ICV, etc. If that doesn't clear it up you may have to take it to the mercedes dealers for a diagnostic....then decide if you want them to fix it, or take their diagnostic and fix it yourself.

Good luck.

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JR
1986 Gold 300E 4Dr automatic