My 1986 300E was still sporting the original Becker tape deck.
It actually stopped working entirely a year ago, but I just got around to replacing it.

I was not able to locate any sort of custom wiring adaptor harness for these models. Crutchfield had some adaptors for the speakers, but they were $13 + shipping, and I was still going to have to solder them onto the new unit.

This was my shortcut:

I opened up the old Becker, and cut out the male power connector plug and the speaker connector plugs from the back of the unit. I simply cut away whatever circuit board, wires, and plastic I did not need with diagonal cutters, and got down to just the connectors with bare pins on the backside.

I then soldered all of the wires on the new unit to the appropriate prongs on the old connectors. Heat shrink tubing and liquid electrical tape was used to wrap everything up tight.

The advantages:

1) All wiring layout and soldering was done inside the shop instead of inside the car. Easier to work at a bench, better lighting at night, and it is still freezing cold around here!

2) The factory connectors are really nice compared to the typical crimp-type that are commonly available.

3) No modifications to the factory wiring. No cutting, splicing, or hacking into harnesses. Plus, when my car is an antique, someone could easily slap in an original Becker for extra points at the Concourse d'Elegance. (Yeah, right!)

I stuck with the old speakers for now, so I just used one channel, and kept the factory fader. I had thought the front left might be out, but it was something with the Becker, because it works fine now.
Anyways, the speaker are now the weak link, but I am loving the MP3 capabilities of my new Sony CDX-MP40 ($159 net at pcrush.com).

Also, I wired it up to use the 'Accessory' circuit as the main power supply instead of the 'Battery'. One dead battery, and that standard German 'convenience' of having the stereo stay on when the ignition is off will not look so great anymore!

Best of luck with your project.

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1986 300E 5-Speed 240k mi.