I have the RioCar in my '97 Twin Turbo, same problem. Ended up using a 12 gauge wire from the Rio mounting sleeve to a solid grond on the car, whine problem solved. Once the whine was gone I did have slight but noticable engine noise which I got rid of with a noise filter on the constant power lead.
BTW, junk the Becker and get a tuner module for the Empeg, I had nothing but trouble with the factory Becker unit and CD player/changer. I'm a happy camper now.