It's worth checking the ground strap, at least annually. If the engine is reasonably well insulted, any metal connection will try to act as a ground. This can get rather spectacular. On my Healey, for example, if the ground strap is disconnected for any reason, the choke cable tries to become the ground. That can be disastrous when you try to start the car. I'm not sure what, if any, metal parts on the MB make continuous contact from engine to ground.