How do I change ignition timing on 300E?

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
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I believe your car is a distributor type... if so, you just need to loosen up the hex bolt at the base of the distributor and turn it... The sticker by the radiator will tell you how to set the timing advance.
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