Here's what I'm doing
I purchase a wiring "harness" from Crutchfield.com for my 1980 300D. I believe it is the exact same one for your car as well. If you go there and click on "What fits my car," you should find it. It costs $15.
But anyway, it contains one double-ended plug that you put between the connection at the cigar lighter. It has a red (switched power) and black (ground) wire coming off that you use to attach to the head.
The rest of the "harness" is actually composed of four speaker harnesses, one for each speaker. In my Becker radio there are only two speaker plugs, because they go to the fader and then to the four speakers, but I think I remember hearing someone else say that they have four plugs in their radio, so I think you might be able to use all of these harnesses. I, however, am going to rewire all of my speakers directly and not use the harnesses (except for the red and black one).
How I'm going to run the speaker wire, I don't know. If you can use all of the speaker harnesses, then you won't have to run any wires except for the constant wire. I met an electrician and he said the best way to get a constant would be to just put the end of a wire between the end of a fuse and it's holder (one that has constant power when the car is off).
If you want to do it without the harness, then you can just do the constant, the antenna jack & antenna wire, take the red switched power from the cigar lighter, the ground from the back of your current radio, and just speaker wire to the new head unit.
Oh, and also, if you're blue remote/antenna wire on your new head is a female quick connect, and the antenna wire from the car is also female, you need to get a double male connector (if you can find one), or just wire-nut them together I guess.
Sorry for the long response!