The pre-2007 audio system is totally non-standard.
The hole in the dashboard is not DIN shaped. The unit you see is called the Infotainment Control Module. This ICM is the box that links the three different computer networks in the car. The I-bus, the P-bus, and the O-bus. Without it the car won’t go. The I-bus controls all the car info, like the status of the locks, security, lights, seats, etc. The P-bus controls the engine and ignition. The O-bus is the optic fibre network and is solely used to control the audio.
The ICM knobs and buttons control the audio. The tuner/amp is hidden behind the aircon panel. The CD player/changer sits below the tuner/amp. All units are powered, but the sound and switching control are by optic fibre. The four speakers are wired to the tuner/amp.
If the car has 7 speakers, the extra three are fed from an amp under the passenger seat. Front door speakers and centre dash speaker. The amp is connected to the optics for sound and data.
If the car has 13 speakers, there are two 6 x 9 inch speakers in the rear shelf, with a further amp to power them, and four tweeters in parallel with the original four speakers. This amp is also connected to the optics.
A conversion to standard wired audio can be done after a fashion. The CD player is removed and replaced by a single DIN unit and all the speakers are connected to it. It is a completely standalone system. The existing stuff has to stay in place with at least one speaker, possibly the centre dash speaker, left connected to it. You need this to keep the car legal because all the indicator clicks and all the warning sounds are generated in, and played through, the existing audio system.