I have a 1987 300E and the timing is a bit advanced.

I world like to back it off a bit.

I heard there is a trim socket with different value resistors?

Where is this located?

On the drivers side inner fender where the ignition parts are mounted I
found a round plastic cap.  I unscrewed it and there were 9 metal contacts,
numbered 1-9, but I think this is for diagnostics.

Could someone explain where this adjustment socket is and how I can find it?

I did see a small rectangular box also in the inner fender with a think wire
coming out if it.  I tried to remove the cover of the box but it didn't seem
to want to come off.

Thanks