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On some it's easier to sell the car than pull the CDM.
The Audio section will have some information on removing the head unit and how to "bypass" the amps in the speakers. Also, the radio receiver is a separate component on the later C4's and to complicate things further, they are in different locations.
And as mentioned, knowing the year of the car helps in providing specific answers.
to make an aftermarket unit work correctly you will need to bypass the Radio rec. and preferably replace the complete speaker system. There is a wiring harness that you can order to accomplish this.On some C4's you will need to wire in 2 .5 Ohm resisters.