[DOCUMENTATION]: Digital BT/AUX trough I2S in Volvo V50 (probably most P1s) | SwedeSpeed

This post is about adding aux to a Volvo V50/S40 sound system. Probably works on most of P1 as well. I am aware a similar post exists here, and that a big part of the info I am going to present here is in there but 1) it is really messy and difficult to find and 2) the OP used a method very technically different than this one. Please excuse if some expressions are not understandable, English it is not my native language.
Also, I am going to publish everything I design here with schematics and diagrams and code, but I would like to sell the boards when I land on a design. Please everyone who wishes to contribute beware of this and someone from staff tell me if I can “promote” my “product” here once done. More like my building service, since you will have the sources as well.
My first mod was also tapping into the amplifier into the trunk: finding the analogue lines that come from the MOST chip and tapping audio into that. It sucked, as you had to run a cable until the front of the car, build a circuit to “treat”(dc biasing + isolation) the audio from your phone and it picked up some noise and hum.
So I decided to go the digital way, with a Bluetooth module inputting an i2s stream instead of the CD player unit.
My approach was to use two of this boards that break down the FPC connectors into pins, to see whats going on.

It looked like this, as janky as it gets.

We have a 18 pin FPC cable coming from the CD reader (CD7/mecha) to the main board (MB).

Labeling is pretty much the same than with our model with the biggest difference being the pins 1,2,3 carry an I2S signal instead of an analogue one.

How did I figure that much out? Well the LRCLK, CLK and DATA marks on the CD reader kind of gave it away.

There is some more differences, check the following table carefully if interested.

The MB talks with Mecha with a serial protocol that looks as follows:

The i2s lines come from the MN6627482WA, which is a digital signal processing chip. Unfortunately the serial comms come from the 8013FGCE19 microcontroller, so the serial protocol has to be decoded manually if we want to make use of it.
I would like to be able to play audio from Bluetooth without a CD spinning, but without any CD the radio goes to mute. If we disconnect the serial communication lines from the MB to/from Mecha, we get an error into the display and the radio mutes, however pressing the next song button gets rid of the mute! I have not been able to replicate this behavior again. Will try again with the new dev board.
Furthermore when pressing the next button again, with the serial comms cut, the small mute that happens between CD songs still happens, which tells us that 1) the mute does not come from the mechanical unit and 2) that is a fixed time mute, without any regard on how long it takes to play the next song. This means we cannot get rid of this mute from the ribbon cable.
Just as a side note, the I2S line do not go directly to the mainboard input switch, SAA7706H, they go first through a resampling chip, located just next to the most controller. This is the chip AK4121VF, but I do not know to which output frequency is set. Also if you do this on your own be sure that your I2S chip supports Right justified data.
Then I should describe the behaviour of the power pins. Note that the pins labelled 3.3v are actually 5v. The 5v are always on with and without the car key. When the mecha pulls up pin 11 – ACC_ON the 8v rail is turned on. I am unsure how the mecha knows the radio is actually on. I am assuming that it listens to the serial data stream continuously, even with the car key removed. This seems rather odd to me, but seems to be the case, since MTS is 5v high even with keyfob off, would like some help on this regard.
That means we do not have a switched power line on the flex connector without having a CD spinning. Very sad indeed
For now on I am using the 12v switched that come into the head unit itself.
My next step is going to design a debugging pcb, so it is easier to get a good grasp of what is going on with the power lines.
Also I seem to have some problems with the grounding and the I2S lines from the Bluetooth module. First I need to add some capacitance between the LRCLK line and ground to get the thing to work. Secondly disconnecting the ground wire on the Bluetooth module does not cut the power completely, there is not enough voltage to run the thing but it tries to boot and some leds light. I do not know why, the only other thing plugged were the I2S pins.
For the dev board I am going to try to implement a LIN bus proxy, based on the one you can see here. This will handle the right hand side steering wheel buttons. Power will come from the ICM or CD unit 12v switched cables. This is the best solution I can find to get power while the keyfob is on 0 (not going to give up those moments when you turn off the engine and the music keeps playing).
I will gladly take any advice on this matter, and will share the pcb designs when ready. A lot of firmware has to be written as well for this to work reliable, but I hope to get a usable board early. Also I will try to get my hands on some cheapo digital analyzer.
For the time being the dev board will have the following structure, with the comms between BTM331 and ESP being simulated buttons presses, as programming the BTM331 with custom firmware will take a lot more of effort.

Better documentation will follow, as well as documentation on the BT.
Useful resources:
Mitsubishi CD7 Schematics and documentation
Mitsubishi CD9 Schematics and documentation