Might consider having Becker USA install a connector in the face of the radio...small hole in the faceplate just slightly larger than 3.3 mm.
Faceplate hole can hardly be seen...car has been judged several times and no one has noticed the connection mod.
Can be used with iPod or any player having a 3.3mm earphone out: CD player, MP3, etc.
Plugging the male 3.3 end into the faceplate cuts off the radio tuner and passes the player sound to the amp and the speakers.
Becker supplies the male / male cable.
Neat solution with no wires or connectors hidden in the car interior. Faceplates are available to restore to original if needed.
Or another solution: Becker can install an 3.3mm adapter fitted to the radio rear with a cable that can be placed into the glovebox or elsewhere if that is preferred.
Here's the modified faceplate. Look just to the left of the last AM button.
Richard M, NYC