I used this connector.
iP-BKR2 iPod Adapter for Becker and other Radios with "AUX" Menu Setting :: iPod Adapters & Interfaces
I chose this one so I could use it with my Ipod.  My iphone uses the lightning connector so I also have this adaptor. 
 
Lightning to 30-pin Adapter - Apple Store (U.S.)
I'm very pleased with the connector, and installation was easy.