sl 600 rough idle vacuum leaks?
The door locking system on the sl 600 is not connected to the engine in any way. However, you do have a vacuum leak in the lock system and it needs to be fixed or the pump will wear out.
The wiring harnesses on these engines are often faulty and the wire insulation breaks down into dust. It is quite possible the top harness that goes to the fuel injectors is bad, causing weak current and thus low fuel flow to one or more cylinders. These V-12 engines are normally silky smooth. I've heard MBZ is encouraging Dealer to 'good will' replace the bad harnesses on cars up to 7 years old.
Otherwise, it's about $1000 to replace the top harness.