I actually used a Technicpnp stereo 4/6 harness when I upgraded my base stereo.
However I would say it was very difficult to install behind the dash, there is only just enough space in there and you need to use considerable force to push the head unit back into position and somehow push the quadlocks down into a small gap.
If I were doing it again I would use the Eton solution below.
https://www.amazon.co.uk/ETON-F-Seri.../dp/B01BQYZ8S2
This will allow you to pick the high level signal up from the underseat subwoofers and run to an amp. This amp would then drive your subs and your front woofers independently.
Your rears would remain powered by the head unit. Although you could make a similar wiring to run to an amp that tales high level inputs for the rear as well.
The amp I use is the Eton DSP CAN 8 and the quality of sound is unbelievably better than base.
It is a full processor and has a 8 channel output, I only use 6, but i did toy with the idea of adding a centre channel.
In the end due to other forum comments I did not install the centre speaker.
The processor takes the high level signal, flattens the EQ, then for each channel you can set high and low shelves, peaks and equaliser setting via their software.
There are 5 user programmable sound stages on the DSP, I only use one. But if I had the time and skill set I could have 5 different sound settings including time delay etc.
I actually was so impressed by the Eton DSP that I purchased a set of Eton 195NEO subwoofers and a set of the B150T front speakers. Which are very well manufactured.
They are Germany company and take pride in the fact their products are made in Germany.
If I am honest I copied an install from the US (without the 3D printing). All credit to Phil, a great guy who even gave me some good advice.
See below
http://www.diymobileaudio.com/forum/...-jl-audio.html
I would say you can probably make a similar cable to the Eton by purchasing the connectors from aliexpress (cheaper than BMW dealer), but you would need to identify the correct wiring and whilst it would be a nice project which would be cheaper it depends on how much time you have.
I hope this is useful.
I really should post a build thread......