I am unsure what the factory fits however when our 4/4 was built we optioned it with the stereo installation kit and selected our own stereo head unit when the car arrived in Australia and had it installed by the local dealer. We went for a Pioneer unit with single CD, remote control and bluetooth so we have hands fee for the mobile phone and can stream music from the iPhone without needing cables. The speakers fitted by the factory were Clarion units although I'm thinking of upgrading these. Of course once you get over about 80 - 90 kph the quality of sound is all a bit academic as you cannot hear much, particularly with the side screens removed.
I use a Sony head unit as you can get a motorbike remote which I have at the end of the transmission tunnel -- this saves either fishing around for the normal remote or diving under the dash. I also have the rear speaker outlets wired to a 3.5mmplug which I use to plug in motorcycle speaker earplugs -- removes the wind noise and lets you hear all road / car sounds with the radio at a low level! See pic re remote location
Hello, In further times MMC also mounted Blaupunkt radios. I tried a VDO radio ravigation system but the navigation part was not loud enough. I would not recommend a CD system because the "feedback" from the suspension causes droupouts. I mainly use the Radio as a booster for my smartphone navigation software. Therefore I bought a JVC stereo with Bluetooth and MP 3. Very lightweight because of the missing CD drive and good to handle because of a remote control. The price was around 80€ (with 8GB USB Stick) what seems quiet ok for me. My "own music" is on the stick and 8GB lasts for many CDs if they are MP3s. But as said before I do not hear much music but it is a fine booster for the smartphone's navigation announcement. They are otherwise not loud enough especially on the Autobahn. If wanted I could publish a few photos.