Various differnt Routes.
1. Keep the Original Head unit an Purchase a Connects2 Unit. They make lost of differnt types.
Basically as the Standard Head Unit doesnt have a AUX input on the back (well mine didnt) you have to have a FM adapter. This is not the same as a Transmitter. It Connects 'INLINE' with the antenna input on your Radio, so Almost perfect Sound.
Connects 2,Car Audio integration specialists
2. Connects2 CD changer Unit. If you dont have a CD changer in your Alfa, then you can purchase a Unit that Emulates a CD changer and plugs into the back of your standard Headunit. These have Pure Line Inputs, so sound will be the best. It will plug into your Iphone/ipod via the USB dock Connection, so you can control it via the Headunit/Steering Controls.
3. Purchase an Aftermarket Head Unit, and a Dash Insert. Alot of Head units now have AUX cable inputs on front and back, so simple Headphone Lead into it. However IF you want to be really POSH, and wow people, get a BlueTooth Head Unit. (Sony ones work well). Connect your Iphone via Bluetooth and you can Move the iPhone around the cabin wirefree and it will still play the music, Plus you should be able to answer calls too etc....
If you want to use your stearing Controls, you will have to Purchase a Upmarket Headunit and then purchase another adapter to convert the Alfa Lead into the socket for the headunit.
I know the Bluetooth concpet works on most Normal Phones, but Check your iphone first, because i know they limit the 'things' the the Iphone can do over Bluetooth. Best way to check is go in the HalFrauds and play with a Bluetooth head unit and attempt to connect to one.
Hope it helps