i put in a new head unit today, replacing the becker the car came with. i was going by a thread for probably an 85 and had a heck of a time finding the constant 12V.
finally found this thread that got me sorted out: 1982 300D Stereo Wiring Question
the red wire is ignition switched and the constant 12V is one of two wires (another, fatter, red one) in a sheath connected to a two port female jack. the jack itself will just pull far enough out of the opening to work on it. slit open the sheath, clip the red wire and that's your constant 12V.