I have a nice Sony head unit with a small tft screen. I got the lead which connects it behind the dash to enable the steering wheel buttons to work. From what I can gather there are leads for most manufacturers of head units. Mine are made by autoleads. http://www.autoleads.co.uk/home.html
I have my sony running two amps in the boot, 2 12" subs, and all speakers replaced in the original pods inside the car. The amps are mounted behind glass with lighting in the boot, and the sub box is moulded into the boot. Looks lovely.