Wiring kits... an interesting observation.
At the time these beauties were manufactured, the nature and type of 'head units' was relatively basic.. but some way removed from the valve systems.
Original wiring saw a 'switched' power source and if you were lucky, speaker wires for 4 corners, being fed from the head unit to a 'front-rear' fader. There was also a trigger for the electric antanna.
Given the age of the cars today, one can really toss the old speaker wiring to the dumpster as it will have aged enough to cause deterioration in signal. The other thing is that the old 'fader' built into the centre console, is a source of noise. That said, all head units today have in built fader controls, making the MB item redundant.
My recommendation is to replace the original wiring for the speakers, in itself quite simple to do once the front knee rolls are removed. In case of the rear, when replacing the original speakers, one would need to remove the back seats to get the parcel shelf out anyway, so at that time, it is also easy to run new wiring for the front speakers and it doesn't require removal of the centre console.
New wiring, removal of the original fader and then run a new 'unswitched' - feed from the terminal block [located under your battery tray] and don't forget to add a fuse link, to accomodate for the elctronic 'memory' of preset stations. All too easy. Just takes time for the preparation. Then if you need to add in amplifiers etc, you are right to go.