If you buy the cable that plugs into the cd changer port (if Dension makes one for your radio), then you can control the music from your head unit using the cd changer controls, i.e. play, rewind, fast forward, etc. You can only move between songs, though, within a play list. If you want to listen to songs in a different play list, then you need to do so via the ipod.
If you buy the cable that plugs into the AUX input using rca jacks, then you'll have to use the ipod to change songs.
Via either method, you can still use the volume control on the head unit.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".