You're probably looking for a lead that plugs into the CD changer socket on the Clarion head unit, and lets the iPod pretend it's an autochanger. From memory, it's almost as expensive as a new head unit.
I had the same idea as you - wanted to stick with original equipment as much as possible. But the difference sound quality and usability (why oh why did Alfa/Clarion think that two separate buttons were a more intuitive volume control than a knob?), plus a front panel aux in and a bluetooth connection for hands-free mobile use, and I have to say it's a sacrifice to authenticity that's well worth making.