The radio harness adapter is still available at Crutchfield. It's very common, pretty much All MB (and other German brand) radios from the 70s (and maybe earlier) through the 90s (and maybe later) used the same power socket.
https://www.crutchfield.com/p_120701860/Metra-70-1860-Receiver-Wiring-Harness.html
(BTW, it comes for free with new head units, you can probably find it way cheaper on ePrey)
The W123 may be different, but on the W126es I've done, it was way easier to pull wire to the speakers than to hack apart the factory wiring. A piece of bailing wire makes a nice "fish-tape" to pull wires to the front speakers, the rears can be run under the carpet by the doorsills. The fader only fades the positive side of the speaker connections, the negatives are made up through a splice. The way it's all connected means that if you tie into the existing speaker "connections", the negatives are bridged for each side without hacking up and redoing a bunch of wiring. Annoying.
The biggest benefit of pulling your own wiring is that you can pull some 16AWG speaker wire to the speakers which will give you the full power of your modern head unit and full individual control of each speaker for any sort of audio enhancements it provides. The tiny wires already present are fine for the 10-12W that the original radio produced, but modern radios produce roughly twice that RMS and it is noticeable especially in the rear speakers.