Becker radios USA has an adapter for older Europa, Mexico, and Grand Prix radios (even tube / valve models) that simply plug into the back of the unit. I just received one it it performs beautifully. You can also send your radio in to have it modified where they will install a mini jack in the faceplate. The cost for the cable is $40, the radio mod is $100.
http://www.beckerautosound.com/Classic/classic aux input info.htm
Mark
http://www.beckerautosound.com/Classic/classic aux input info.htm
Mark