The X/11 plug is on the drivers side inner fender, a round multi-pin plug with a cap that unscrews. You can measure the on/off ratio of the oxygen sensor and see if the fuel mixture is at the proper specs. To my knowledge, it's on all '80s/early '90s MB models.

Steve Brotherton wrote a great article about this, it's a great read.

http://www.peachparts.com/Wikka/EngineControls