Simple,
Do what I plan to do in my 94 E420. Get an alpine unit that plays MP3s. You can then burn a CD with TONS of songs. I'm getting the CDA-9815.
It's $348 here
http://www.1-877camcorder.com/cda9815.html
Oh, and if you decide to get XM radio for it, let me know first.
Also, I can let you know what I'm doing regarding rewiring.
Willy