Leave the radio as is... better keep it original than put in an aftermarket radio.

The 1990-1991 560SEL's with ultra low miles sell for the minimum of around $10,000.. most in the $12-14k range(50-75K miles). A cream puff, fully documented black/grey 560SEL recently went for $18k and it had around 70k miles on it. A member once had a 560SEL with 20k miles on it... sold for $20k.
The more original the car is, and more documentation you have of the car.. the more its worth.