The sound system in this car is an upgraded $2000 aftermarket system.

The benefits are great sound, and a more modern, versatile radio (head unit) up front.
The drawbacks are slightly less trunk space, and more complicated components.

I swapped the old stock cassette deck head unit with this Alpine CDA-9855 in order to have the ability to play CDs, have satellite radio, and have better sound.

I have the Alpine HU hooked up to two aftermarket Blaupunkt VA2100 amplifiers. These sit behind the carpeting in the trunk and reduce the available trunk space by a bit.

One amp feeds two JBL subwoofers mounted in a JBL "Stealthbox" in the roof of the trunk. The other amp feeds into crossovers for 4 CDT Audio aftermarket speakers that are up front. These are nice speakers with ceramic drivers, which produce a softer sound. They are a 5.25" component set.

Everything has been wired carefully, with special attention paid to grounding to avoid system noise and ground loops. There is a Sirius Sat radio receiver behind the glove box. If you want Sat radio, just call Sirius to activate service.

The stock amplifier, along with the old cassette deck and an old CD changer that was in the trunk do not come with the car. As with many BMWs, don't expect any AM radio reception.

Part of the audio upgrade project was to install significant amounts of additional soundproofing in the cabin and trunk. This reduces external noise and makes the cabin quieter, a nice bonus!

The system may take a little more effort to maintain than the old cassette deck, but you will be rewarded with great sound and a CD player up front, and most of the expense has already been done for you. If anything ceases to function, just take the car to your local car audio shop and let them fix it for you. The system is very solidly built, and they should easily be able to handle any repairs it might need.