It depends on the head unit. I know that some old Pioneer Head units are easy to hack. They have a 3.5mm jack input for audio controls and you only need a combination of resistance value for each button (vol up, down etc). Some others as JVC are more complicated as they need pulse modulation so you need to find or hack the code from remote control with reverse engineering. I dont know about new head units we have to do some search..
First of all you need to mount some switches on your wheel.and get or retrofit (if it's possible) the cable reel so that you can get 2 extra wires from it. That's what I am also searching for :)