Here is my cheap solution of controlling Sony/Pioneer which support wire remote control(sorry my bad draw but i`m far away from electronic guy):
The upper Ohms symbols are for original resistor values on each button on my MK2 Xantia. Then follow the original diagram , my working idea diagram and last one is that one what you can find on every audio forum to control that two brands stereo(or that ones which can)
And that ones are my first tests:
The upper one was ok, but i experience small problem closing ok the button control body cover. The second one, the resistors was soldiered UP as image show and that help they to go inside of the plastic covers just perfect with no issue.
Then in my case somehow that film type cable and the black plug on it start to give me some hight impedance and that`s why i remove all that and just made small board with all resistors on it. After that with 2mm drill i`ve made holes close to the buttons and cut unnecessary connections like that:
Just to mention that the 2 small buttons on top in that picture are : left my new one that was suppose to put instead of original, and the right one is the original. Why is out? Well... because Murphy come around and during the soldier-unsoldier procedure some of that button leg unsoldiered and resistance start to float form 60 to 180kOhms .. Mention this to be careful
Now my Pioneer DEH-1500UB work just fine controlled with the steering wheel stalks
Hope that info will help someone else to break his car he he..