http://www.c2club.co.uk/showthread.php?20985-Guide-How-to-enable-the-CD-Changer-Port-for-use-with-an-iPod-Adapter-with-RD4-Units

I thought i might share some info with you all, how to enable the CD Changer port on an RD4 Stereo in a Citroen/Peugeot.
I did a lot of research and still couldn't nail down the correct options to enable, how to use Lexia or a definitive answer to why the port doesn't work from factory, people were waffling saying that the options are different between cars, that is rubbish, we tried it on a DS3 and the stereo is the same on a DS3 as a C2!

So i had access to DiagBox with Lexia over Xmas, and so I have made a little guide for you all

You need to enable the CD Changer Port, as well as the CD Changer Display function, otherwise the source details will not appear on screen.

1. Connect your CD Changer device, or iPod adapter unit to Port C on the rear of the head unit, the closest free port, to the passenger side of the car when the stereo is pulled out.
2. Connect the Lexia Dongle to your car's ODB Port, and your Computers USB Port and ensure that when you run the DiagBox software, it finds your car's VIN Number once you have selected your cars model in the software.
3. When the tool has located your VIN Number, enter the relevant RPO number (this can be any number and is used by dealers only), i used 12345.
4. When prompted with your 3 options, load the full Lexia package not ScanTool.
5. Once Lexia has loaded, and connected to your car, you will be presented with a heap of options.
6. Go into Diagnostics
7. Select Global Scan (this will attach it all ECU's in the car, and tell you if they have communicated with the lexia tool of not)
8. You should have a list of your car's ECUs, with two columns, Y or N for "Communication", and a Y or N for "Faults Detected"
9. Double click on your Radio item in the list
10. Now go into Configurations.
11. You will now be presented with the following screen:

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12. To enter configuration mode (by default you are in view mode) press on the F5 button or double click on the Aux1 option to enter configuration mode.
Now you first need to enable Aux 1/2 before the CD Player option at the top will allow you to choose "present" otherwise it will only have the "missing" option choice.
13. Double click on each Aux input, and change these setting to "classic.", press the enter button * to go proceed and back to previous screen.
13. Now you can change the CD Player option by double clicking, and selecting "present" instead of "missing", again, click enter (*) to accept changes.
14. Once you are one in this screen, click the Configuration F5 button at the bottom to submit final changes to the ECU.
15. Now go back, using the Back Button F1 a few times to get back to the list of car's ECU's we did in the global scan.
16. Double click on the BSI ECU and again, enter into the configuration options, here you will see these options:

Untitled2.jpg

17. Now change the CD Player Option to present, this will enable the CD Changer display setting on the car's BSI ECU, and enter to proceed.
18. Press Configuration F5 to save final changes to the ECU.
19. Press Back F1 to go back to the main ECU list from the global Scan.
20. You're done! you just need to go back out of the software, and exit, turn igniton off and back on, and you should have the CD Changer port enabled with your ipod adapter device.