I can say to you, have fun. This is the reason that I haven't replaced the head unit in my 2007 Chevrolet HHR, the stock one is too tightly integrated with the rest of the vehicle electronics and replace it (although possible) means losing:
* adaptive volume
* turn signal clicks
* warning chimes
* steering wheel controls even for cruise/trip odometer (iirc)
However, there are companies that provide interoperability adapters for factory systems, Metra comes to mind. They make a GMLAN integration module (GMLAN is a protocol that runs on top of CAN bus) that reenables warning chimes/turn signal clicks/adaptive volume for late model GM cars. But it costs like 200 dollars IIRC.
I have no idea if one exists for Citroens.
I hope this helps.