modding out a oem stereo

I am about to take my touchscreen Pioneer stereo out of my 2006 Mustang and reinstall my Shaker 500 with built in 6 disc CD/MP3 changer and a AUX button that does nothing. Inside there is a main board with amp, a interface board with the buttons and small screen and a board with the DSP, chips to drive the motors and cables that go into the CD changer. I am also going to mount a Android tablet on it with a bracket that is held by the CD slot. The DSP control board connects to the main board via a tall 20 pin connector. Originally I just wanted to tap into that connector on the pins that send the audio on from the CD down to the Amp and plug that into a DAC that would then plug into the tablets usb port. So to start off how would I detect which pins those are? Hook it up, plug a cable to ground and touch the other cable to each pin until I can hear the tablets audio over the CD music? Once that is working I would like to put a auduno board inside and plug all the CD control cables into it so it would send a command to the tablet when a button on the stereo is pressed. That way I would still have physical buttons to fast forward tracks, etc. Any help on what signals I should be looking for or the best way to get started would be greatly appreciated.

No, that kinda defeats the purpose. I really want to be able to have the advantage of a touchscreen android tablet along with full use of the tactical buttons and knobs. Plus I want to do it myself. OK I also don't want to shell out $100 for what could be $3 worth of electronics. It appears that no one that has ever bought one of those has opened it to see what is inside other than the people that make the knock off versions for $70 instead of $100. I also know that if you use just the right resistor to connect 2 pins in the 16 pin connector to tell the processor that a AUX device is plugged in then you can just solder cables to that connector. Once again the only people that know that information seem to keep it to themselves so they can mod out radios and sell them on eBay.