I originally found these pics on planet Renault. The Cabasse Auditorium head unit does have an aux-in function, but it's not at the back of the head unit - in fact it's in the boot!
Because you have the 6-cd changer in the head unit itself, it has been necessary to locate the electronics (amplifier and tuner etc) in a unit elsewhere in the car. On the Grand Scenic it is behind the right hand side cover in the boot area. To get to the unit, remove the luggage cover, remove the torx screw on the luggage cover plastic support and the other torx screw at the opposite end of this piece of plastic. The carpeted side panel can then be removed and behind it (just near the backside of the petrol filler cap - you will find the electronics unit for the cabasse.
The pictures i've uploaded (from planet Renault) make it pretty obvious what you need to do (even if your french is not so hot)
Take a PC cd-rom audio cable - and with a little help from a soldering iron, connect it up to a long enough cable that terminates in a 3.5mm jack socket. The cd-rom end needs to be plugged into the red 15-pin connector; on the following pins:
27 - Common Ground
28 - Right signal
30 - Left signal
You then need to root this cable through from the boot to the dash. I found the easiest place to run cables was under the footplates (tip - start removing these from the back of the car forwards, and be careful - they crack easily and are expensive!!!!)