Of all the different systems fitted to the P38, the DSP one is the worst when it comes to wanting to fit anything aftermarket. As most of the processing was done in the DSP amp, there's only a stereo output to the amp and a separate output for the sub. You'd need to bypass the amp so the left and right feeds to it are linked to the front speakers and run a separate two pairs for the rears. Ideally you'd need to add some crossovers too. Having looked at the work Marty has done and the grief it has caused him, I'm really glad I don't have a car new enough for it to be fitted with the DSP system.
And the biggest thing was that I didn't have the DSP system... when I bought mine, the DSP amp was missing, so it was a case of make something work. I could have gotten away without the crossovers et-al and just fed full range to everything, and it would have worked - but I like my audio to sound somewhat decent, hence making the crossovers.
If I knew then, what I know now about the head unit being the same as for the later non-DSP stereo setup, I would probably have bunged an amp board in mine a long time ago and been done with it!
But even so, the DSP system, unless it's running how the factory intended and the amp hasn't shat itself, then it's just a PITA to do much with.
Just got home and had a read of the wiring diagrams. There appears to be lineouts (audio out L and Audio out R) that may be suitable for wiring into the cd changer as I previously did with the 3.5mm jack on the old car.
So that would make it Audio out L goes to the Green wire of the blue connector on the radio and the Audio out R goes to the Purple wire of the blue connector. And ground to ground obviously. No need to use the sub output as the dsp will sort that out.
Am I right or wrong?
I know it's Heath Robinson but there isn't much choice.
In your case, I would either try the line outputs (they probably won't be much good as they possibly won't be what the level is that the DSP amp is expecting), or make 2x attenuator units up, and use the LHF and RHF output from the android unit, through the attenuators, to the front left/right feeds to the DSP amp.
As your factory head unit works, etc, keep that so it's powered, and then it can do the controlling of the DSP amp, for final volume control, fade/balance etc.
You may need to wire the sub into the output from the android unit, as I'm not 100% convinced the subwoofer is fed internally from the DSP amp - if it was, why would they run the wires from the head unit for it? especially when they didn't run rear/left right from the head unit... so if the sub wires aren't used, then why are they there etc...
If you are using the audio out L/R (D2/D3 on your sheet above) then these will be line level. If you use the wires in the main connector, then these will be amplified. You'd also be better using the FL +/- and FR +/- (white + white/black and grey + grey/black) through an attenuator circuit to the vehicle wiring.
If you wanted to do it 'properly' then I'd rip the DSP amp out, and replace it with something like my DSP replacement board, which has 4x door amps on it - and then wire in all 4 corners from the android unit (thru attenuators) to the appropriate doors. Then the nav unit is controlling all the audio, and you could probably sell the working DSP amp for a bit and cover the cost of the install...