I have a 2003 E500..I would like satelite radio in it. Can it be adapted for that or will I have to get a completely new radio?

It is FAR cheaper just to replace it with an after-market unit. Pioneer, Kenwood, etc. The newer units with have sat, HD radio, Navigation, etc and far better resolution than what originally came with the car. Dash kits are available specifically for that car so the new radio will fit and look exactly as if it came with the car. Even if you could add Sat to your existing radio - the main unit would still be 12 yrs old and lack the features and performance of newer radios.

Also, IF your model has fiber optic wiring (probably not), there are conversion kits which attach to the wiring harness behind the radio to retain functionality. You will not need to rewire any speakers.