I have a USA Spec unit in my car. It just plugged into the head unit where the 6cd changer would plug in. Works great, allows me to control ipod with steering wheel controls. It cost a little more but I found the steering wheel controls 100% worth it
Heres a pic of my setup. Although, the ipods in a different position now.
http://img169.imageshack.us/img169/5228/img203cn0.jpg