I don't know what wire was used for the 12V power but the rear speaker wires run up underneath the drivers side floor mat on my 85 300D.. use that to trace under the console to wire up the rear speakers.

I didn't completely finish the job yet so the fader wheel still works.. it just alters how much power runs to the front speakers.. either full sound or no sound.

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'87 300D new to me
'85 300D Sold
'83 300cd Sold