The Alternatives.....
Replace the Whole System
This is probably the best route if you have the money and the inclination to do so. This requires replacing the head unit with a CD or cassette receiver, amps, and speakers. Lots of work here but is probably the best route for the purists or those into competition. The moths in my wallet gagged on this option.
Replace Just the Amps and Speakers
Some people told me they had good luck with this option. If they were already vested in the BMW changer and head unit this made since. By adding cleaner amps and speakers with better frequency response they got the sound they were looking for. This route was a still a little more expensive and time consuming than I was ready for. This requires adapting the current head unit to the new amps , installing and wiring new speakers. Plus , I still had to add the cost of the BMW changer unit ! The moths were in cardiac arrest.
FM CD Changer
Ok, this was a pretty affordable option at around $300. There are several 10 and 12 disc units on the market for around this price. Installation seemed simple enough. Mount the changer in the trunk, under seat, glove box, etc., and then run a cable to a RF sending unit that transmits the music to your FM head unit in stereo. You will also need to buy an adapter to adapt to the European antenna connector used on the back of the stock head unit. There is usually a small wired display for controlling the changer and showing CD information that can be mounted anywhere you can find a space. Some units also have a remote control. I talked to several installers about how these sounded and was told that it really depended on the quality of the FM modulator in the changer, as well as the quality of the FM receiver. In other words, it should sound as good as or slightly better than your best FM station. This sounded like a pretty economical approach to getting CD into the car, but would not offer much improvement over the stock audio system. However, there was a benefit in cost, not having to remove the stock headunit, and not messing up the interior coordination. The moths were happy and I considered this option very heavily.
BMW CD Changer
This is what got me into this in the beginning. I found that I could order one of these for around $500. This would indeed be the simplest option to add CD sound to the car. I was not sure if this was what I wanted to do. Would I get any sound improvement ? Probably yes by the virtue of CDs’ cleaner sound. However, I was concerned that the same problems that plagued the FM and Cassette sources in the audio system would still be there, so the gain in audio quality would not justify the cost. The FM changer started looking better.