Are you looking to use your steering wheel controls to control your head unit, or are you trying to send commands to the CarPC via the steering wheel controls? I ask, as that is designed to interpret controls from the steering wheel to different manufacturer's head units.
If you want to control the CarPC with the steering wheel buttons, check out the RcJoyCon. In the video on that page, they show it controlling WInamp. I use one, and have set up hotkeys with it, and then you just define the hot keys in RR's KeyTbl.ini