What plugs in here?
 

I recently got an '84 300sd. Inside the engine compartment on the drivers side and fairly far forward there is a screw on cover that resembles a screw on bottle cap. If you remove it there is a place to plug in an electrical plug . There are 7 holes numbered 1 through 7 in a semi-circle around one side and 3 holes in a straight line on the other side numbered 8-10. What is it for? What kind of electronic stuff does this car have (computer etc.)? I also have an '81 w123 240d which has very little in the way of electrics compared to the 300sd. I'm sure I'll have more questions. I would also very much like to get wiring diagrams for the 300sd. Thanks for your advice.

Hello - would you be able to post a photo of the engine compartment section that you're referring to? For the wiring diagram, if you go a google search for '300SD wiring diagram' and then click the Images tab; there are a few examples of it.