Does anyone know if there are any MP3-compatible cd players that would fit into E46? I presently have a standard BMW business CD player.

See if this is something that would work. Went to Tweeter.com and punched in the E46 platform on their "fit guide". They say this fits. Worth looking inot:

http://www.tweeter.com/product/index.jsp?productId=1336152&cp=1124288.1124546