zen_

Unless your vehicle already has an AMI unit installed, you need to purchase and install the whole thing. I bought mine from this seller on eBay after basically being laughed at by the dealer inquiring whether they could get or install it.

http://www.ebay.com/itm/Audi-Music-Interface-New-Genuine-RNS-E-AMI-Ipod-Interface-USB-MP3-Retro-Fit-Kit-/181189508823?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item2a2fbc7ed7
I would also note to anyone looking at royclark's factory install to check your glove box first if you want to keep the DVD / CD unit and install the AMI unit next to it as pictured. In my glove box, the right hand compartment has a back plate in it and lacks the side latches that secure the unit in place. If that's the case in your vehicle it will require some modification and you won't get the 100% factory install unless you're able to find and install latches too. The kit I linked comes with the optical splitter so you can hook up both units.

Install is fairly easy. First update MMI to 3.3.60, below is another link to an eBay seller that sells cheap copies...I would image and upload them, but it's already a sort of risky affair updating firmware, and I don't want to be responsible for anyone bricking their MMI system! Have a place where you can safely leave the vehicle running or hook it to a battery charger in the garage, because it takes hours to do the update. I had to update mine twice to get the bluetooth handset function to work correctly, luckily it didn't brick the first time.

http://www.ebay.com/itm/Audi-MMI-Software-for-2004-2008-A5-A6-A8-Q7-MMI-Bluetooth-Update-Kits-/251264266297?pt=Car_Audio_Video&hash=item3a80847039
The only tip I have for changing the units is bend the stupid keys a bit to get better leverage so they "click" into place correctly, then push the unit out from the back to dislodge it. The retention system can only be explained by looking at it from the side, and it will probably be a PITA the first time you do it. Remember to keep the keys and don't throw your CD player out if doing a single install, because you may need it down the road.

When you've finally got the AMI system installed, then you can use the AMI cable with either a USB or ipod adapter to plug into the AMI unit. Another note for people thinking of using a USB hard drive. One, the AMI system will only recognize the FAT32 file system...not NTFS, not exFAT. This makes formatting a large hard drive a PITA, but there are workarounds for formatting disk beyond the Windows restriction for FAT32. Second, the hard drive does not turn off when the car turns off. It may spin down, but if it has an LED light on it, it will not turn off. If you're going to leave the disk in there all the time, maybe buy one without any LED lights on it.

zen_

Sorry I linked to the wrong ebay listing and can't seem to edit my post, gah!

http://www.ebay.com/itm/Audi-Music-Interface-New-Genuine-AMI-Ipod-Interface-MP3-Complete-Retro-Fit-Kit-/171077071621?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item27d4fcf305

raj99

At this price point, I personally would go with the Dension 500 unit, since that is expandable to various manufacturers, other than just Apple products.

I have such a unit installed and I know others on this forum have likewise.

http://www.dension.com/products

calberto

As noted above, it is not just a cable, but the whole AMI unit that is a direct replacement for the CD changer. After a year I have had no issue with missing the CD changer. I did not do the install myself.